If out of nowhere you start doing 10miles/day for 3 days in a row it's no wonder it hurts. Like you said: you have 12 weeks to build up to 50 miles. Just keep exercising for 45-60mins 3-4 days per week and you'll be fine. Rest is important, don't push yourself now.
Your current speed is irrelevant, it will improve, but not in 3 days, especially not in a row, your body needs time to adjust and recover. In 3 weeks it will be a lot better and with the achieved improvement you can start extrapolating that for the next 9 weeks.
A short ride, maybe just 10-20 minutes at a real slow pace might be helpful for recovery now, try to focus on a high cadence. It is very annoying to ride like that at first, but tomorrow your legs will feel a lot better.
The trust in your own capabilities is important, so set a distance goal per week for one ride, and don't bother about them on the other days. Give yourself 1-2 days rest before that goal, and rest a day afterwards. In 3 weeks that only has to be about 15 miles. Another 3 weeks to 25, and yet another 3 weeks to 35-40, and then 3 weeks before the 50 miler.
Your goal in the first weeks should be modest. 10 miles in an hour without being knackered is a fine result in 3 weeks, so about then it's time to add half an hour, but again: take it easy.
