If you’re ever losing faith in human nature, go out and watch a marathon.

"If you're ever losing faith in human nature, go out and watch a marathon." Kathrine Switzer - Woman's Marathoning Pioneer. An observation that I certainly felt when I went to watch my sister run London in 2009.

I applied to run the following year for my gran who was suffering with dementia. I was unsuccessful in the ballot and The Alzheimer's Society rejected my golden bond application. I knew I only wanted to run for Alzheimer's, so I was forced to wait another year until I was successful in the ballot. By that time my gran had sadly passed away.

I'll never forget receiving my ballot result. It was my birthday and I'd been out for dinner. I came home to a couple of birthday cards and a big A4 envelope. One of the cards was from my grandpa, my first card from him signed simply gramps, no gran. I felt upset and teary as I turned my attention to the bigger envelope. I knew it was either acceptance or rejection. When I realised it was the former, I was so happy. It sounds ridiculous but it was as though gran was there in spirit and she'd got me in the race!

I started training immediately and was soon up to running 10, 12, 14 miles. Whenever I found it tough getting out of bed in the morning I'd think of gran, as though we were going out for a run together. In my head I'd say, "come on then, let's get this over with!", it made it easier to not give up or slack off.

In December, I decided to set up my sponsorship page. I'd heard that the earlier you set it up, the more money you raise. People were really generous but I wanted to show them that I was serious and I wanted to stand out from any other marathon runners that might be competing for my sponsorship money.

My mum started making films when my gran first got dementia. She'd film her holidays and any family events so that gran could watch them and remember things. Gran loved mum's films; she'd watch them over and over again, as if every time she was watching them for the first time. So I decided to ask mum to help me make a film about gran, her dementia and why I was running the marathon. I opted for an Adele sound track to tug at the heart strings, then a homage to the training scene in the film Rocky to lighten the mood. Once I sent the link round to my video, sponsorship soared. People are very generous when they can see you've put in a bit of effort: http://youtu.be/AnRvRGmVJmE

So the training continued well enough until 16 miles when I hit an injury. The run had gone well (apart from it being in the pouring rain), but the next day I was in agony. My right hip felt as though it needed a replacement and I could barely walk, let alone run. I had to go to Liverpool for a week with work so I thought a week's rest might do it some good. I tried running again the following Monday but the hip was still really painful.

I paid a visit to my local physio and discovered I had tendinitis of the hip. I was told not to run for three weeks (the race was eight weeks away at this point). I was gutted. I was worried that I wasn't going to be able to complete the race and I thought all my training had been for nothing, but the physio reassured me that I would be OK as long as I kept my cardio up on the cross-trainer.

I'd never really used the cross-trainer in the gym before - it was to become my new best friend! In the end, I never got to do another long run before the big day. I had to do all my 'long runs' on that machine. The longest session being three and a half hours! I carried on with the physio (certainly not cheap) and read all I could on marathon running: how to pace yourself, how to hydrate, what to eat, what to wear and when the big day came I felt ready and motivated to do my best.

When I started running my average pace was around 9.5 minutes per mile. I decided to attempt to run a 4:30 marathon, which works out at around 10.25 minutes per mile. I had no idea how my hips would hold up and I was fully prepared to have to walk some of the way - I wouldn't have been surprised if I'd have finished in more like 5:30.

Thankfully, on the day my guardian granny was shining down on me. I was at the blue start, pen nine (which to anyone that doesn't know means right at the back of the start). This suited me fine as I knew it would help me set my pace and take things slow. I set off and pressed play on my running playlist, the first song that came on was "Keep on running" by the Spencer Davis Group. As soon as the song came on I had a feeling that I was going to enjoy this one. All I could see in front of me was a sea of bobbing heads and cheering spectators. I had a lump in my throat and a tear in my eye. All I had to do was keep on running!

It's a strange experience running for so many hours on your own but with so many people. I had an inner-monologue going through my head the whole time. From "Is it time for another painkiller" and "Thank God I don't need the toilet" to "How is he running dressed as a giant strawberry?" or "Isn't he getting seriously chaffed wearing that mankini?" But the overall thought running though my head the whole time was, "Wow, I'm really doing this." I never would have thought I'd enjoy the run so much. It was like I was on a high virtually all the way round. Every now and then I'd make eye contact with someone in the crowd shouting "Go on Cathy", and it would give me that extra push to carry on.

Everyone always mentions crossing Tower Bridge but that really was one of the best moments of my race. It creeps up on you out of nowhere. You turn a corner and there it is - beaming down on you like a Cheshire cat's grin. One more turn and I was at mile 13, where my friends and family were waiting with a huge banner saying "Cathy Campbell - Pain Now, Beer Later". It was a brilliant lift seeing them all and according to my sister who was tracking my run, I ran my fasted 5K after seeing them all.

I was a little anxious once I passed mile 16 because that was the furthest I'd ever run before, but so far so good. My pace was on track, even a little ahead of my 4:30 finish time. I focused on getting to 20 miles because then I knew it was only another two until I saw my family again, then it was only four till the end.

Another high point was seeing a sign that said: "In 2.5 miles you'll be part of history". Another lump, another tear! Time to dig in and crack on. I didn't seem to hit a wall throughout the whole race and whenever I felt tired or niggly, there would always be some amazing sight to take my mind off things.

No more so than when I got to the "800 meters to go" sign. In my head I knew there was less than 1K but my body was feeling like it was more like 100K. Then, seemingly from nowhere, ran a man wearing only a G-string with a load of blue and white helium balloons tied to it. He was zig-zagging along the route in front of me and the crowd were cheering like mad! I was in hysterics and I wish I could personally thank him for getting me through that last stretch. If you think of all the people he must have spurred on during his whole race, he deserves more than a medal; a knighthood perhaps!

365 yards to go and you can hardly believe you've actually done it. I looked down at my watch, 4:26 - amazing! I was so happy! It's a slightly surreal feeling when you cross the line because you don't know anyone around you and you can barely walk. Luckily, there was a man next to me who seemed to know what he was doing and he congratulated everyone and we all gave each other pats on the back. The winners’ area is strange. It's very quiet and it feels like you just float through it. I went to collect my bag and made my way out to find my friends and family.

Nothing prepares you for the pain in your legs the following day! I expect mine was especially bad due to the lack of running in my training but whatever the pain, nothing can take away the immense sense of pride and joy I felt, and still feel, having completed the London Marathon. It was such a fantastic day that I will cherish for the rest of my life. I raised £3359 and counting for the Alzheimer's Society and I know that granny Sanderson would be pretty darn happy with that!