A bit of background, I used to weigh 16.5stone and be a size 24 and lost 6 stone to a size 10, after having a child currently 11,5 stone and size 12/14 trying to get back to where I was! It really was a 2 pronged approach where I changed my diet and did some exercise.
Covering exercise first I started Spinning first and then started running after I had built up some fitness and lost a bit of weight, I still Spin now. WIth running it really is a case of slowing it down to a pace where you can hold a conversation, it may be slow but that really does not matter! I have always preferred running outside as I like the scenery, quite honestly I can't run on a treadmill as I hate it and really struggle as a result. Nowadays I do triathlon so am swimming, cycling, running, Spinning and practicing Yoga.
On the food front it is best to plan ahead where possible, take packed lunches and snacks with you to work and keep a food diary. If you wish to try something more structured rather than doing a fad diet like 5-2 I would suggest good old fashioned Weight Watchers where you do not have to fast or give anything up you quite simply eat everything in (structured) moderation.
Most of all I want to say well done for starting, the first steps are always the hardest and wish you good luck with your endeavours. You will get there and it will be worth it!
I would suggest to start by walking up the hill then running at the top, once you are comfortable with this start to run part of the way up the hill and the walk the rest. Over time slowly increase the distance you run up the hill and then one day you will run the whole way and it will be a good achievement. It doesn't matter how slow you go the aim is to take it steady and build up to it.
I know that some of the bigger events down here in the West Country such as the Bristol 10km and the Bristol Half Marathon have wheelchair athletes. I hope they are not just elites and cover all abilities as the rest of the race does. Is there a wheelchair athletes association who could help?