Another vote for the Horizon Elite 507 (12mph, 12% incline) also bought from Fitness Superstore. Had one since 2007, just replaced the belt recently (it's also a thicker and better belt (4.4mm orthopaedic) than a lot of other treadies in that price bracket.
Shortly after that unfortunately the piece of metal where the incline motor attaches to the frame broke, I called the manufacturer (very helpful and proactive), the tech asked me to take a couple of photographs, he called the next day to say that they would replace the machine outright. So we have now got a brand new model, just as good as the old one. So although we had a problem with our original one I would still highly recommend it as it worked very well up to that point and the customer service I received throughout was excellent.
If you do get this model or any other with a cotton backed belt make sure you get powder for lubricating the belt and not liquid as the liquid wrecks the cotton backing, thus killing your belt.
I only wear shorts or full length tights (tights in the dead of winter only). Under the tights I'll wear a pair of lycra shorts to keep Mr Happy cosy and reduce wobbleage. more for my comfort than anything else. I don't really get the shorts over running tights combination, if it's too cold wear some underwear underneath the tights, if it's for fear of a bit of a bulge then really nodody cares, we're blokes we've all got a parsnip and prunes Each to their own I suppose.
Not that I think that what the OP is doing is a good way to go to train for your first marrathon but I've got some personal experience here. I ran my one and only marathon to date in 4h20m on the back of a 6 week timeline, including a weeks rest at the end, so 5 weeks of actual training. Before I started training I had never ran more than 7 continous miles. However, I was playing competitive level squash (3-4 times a week plus training), so had a good base of fitness and being able to push my body, but obviously not the experience of time on my feet running (totally different fitness and aches and sprains involved).
I managed to get 4 long runs in, did absolutely no speed work or anything other than regular running. My 4 long runs were 10, 14, 18, 21 - I consciously slowed down for the 14,18 and 21. Race day was fine up to about mile 22, I struggled over the last 4 miles but just plodded my way through it, was glad to have done it but definitely wouldn't recommend it!! Have subsequently managed a 1h38 half on the back of 2 and a bit months proper training including a variety of training runs up to 40mpw or so, much prefer this approach oddly enough
Would like to try again and work towards the 1h30 and also have another crack at a marathon and do it right this time......
So in summary it can be done but it's going to hurt, as long as you put all notions of speed out of your head you have a much better chance of getting round.
I recently had to replace the belt on my treadmill and got a good price from Expertfitness (expertfitness.co.uk), mine was an orthopaedic belt as well so it was slightly thicker than normal. Just in case you want another place to get a quote from.
We've got a Horizon Elite 507, been very happy with it, had it for about 5 yers and just recently I replaced the belt on it. It wasn't cheap when we bought it (c1000) but you get what you pay for. I wanted something with a longer deck and the ability to go to 12mph rather than the usual 10mph of the cheaper treadmills.