I am 69 years old and will turn 70 in July. I'd like to go for a 70 mile ride on my birthday. Right now I can do 20 miles with no issues. I don't know if I can go much more as I have never done it. I've been biking for about 12 years now. I weigh 293 pounds and my bicycle is a 2012 Raleigh Sojourn. What training should I start to achieve the 70 miles? I live here in "low country" South Carolina and it is all flat terrain (very flat).