Go for it! I'm 50 and at the start of the year 230lbs and only a rare recreational cyclist - decided to do the Etape and set about training for it. Did a reasonable amount of training but not in my mind fanatical, about 2500 miles in 6 months, very important to include a number of 100mile plus cyclesportives - I did three plus the 'tour of Ireland', a four day cycle around Ireland. Also, mainly as a result of training am now 210lbs.
Did the etape, and although I was worried about being swept up was well ahead of the wagon when I reached the final climb up Hautaucam. A fantastic experience and definitely achievable if you take it seriously and do the miles beforehand.
I got my R3 six weeks before the race and it was great. Being my weight I am always going to be slow going up the hills but I discovered a latent talent for coming down mountains which I was unaware of.... while everyone else was taking it very, very slowly (it was raning and pretty wet), I and the rock solid cervelo swooped past what seemed like hundreds cyclists with absolutely nobody passing me in the 25 mile descent (in marked contrast to going up the Tourmalet when it seemed like the world and his wife overtook me)! My Garmin tells me I was hitting 52mph at times.....
If you need more encouragement have a look at my blog: www.michaelmahony.blogspot.com meant for a very limited audience so forgive the parochial style but it will give you an idea of the weekly mileage I did and how I approached it, also, I have some links to much better blogs which gave me a lot of encouragement and training techniques.
Michael