I think the former posters have said it best. That being said, there are hobbyjoggers much slower than those you'd find on these boards that do marathons with their highest mileage week being like 50. Wait until you're not at a crucial time in the training season, and please consider building up to it via gradual increase in mileage if 20miles is truly your goal. As far as pace, if it's just distance you're worried about, just start at a shuffle (seriously, don't even care about the pace) for however long you feel should pick up the pace alittle. I've completed 14 mile runs this way, start at 10:30 mile pace, finish the first mile in 8:30ish, and finish the last three miles in 7:30pace.
You can do whatever you put your mind to, thus use your mind and realize that you'd be better off gradually build up to it if you wish to not get injured. Training Adaption = Stress (Proper Workouts for your current skill level) + Recovery (Good sleep, going easy on the junk food, stretching, foam rolling, etc.).