I definitely think it's doable (won't be easy!) though the wheelchair will be important, you'll definitely want to do at least a few training runs (10K?) with your friend in the wheelchair to see how it goes. I like your plan to train for marathon, but pushing a wheelchair will add different stresses that could mess you up, I'm thinking lower back for example, you might do a 10K early on pushing your friend to see where unexpected stresses develop so you can then prepare your body a bit for that.
11:30 is a pace per mile, which likely means nothing to you, but it isn't super fast in and of itself if you are the runner type. I've never been a runner, broad shoulders (top heavy) and ~215 pounds, but I tried a few marathons in the last year and was able to do a 10K in less than 50 minutes (~8:00 mile) and a half marathon (21K/13.1 mile) in under two hours (~9:00 mile), but the wheelchair is absolutely a game changer, so you really need to figure out how that changes the equation, and the sooner the better.