With age comes a longer period of training time to get up to even half of where you once were! I know because I'm 63 as well. So you have train longer than someone half your age, that should not have come as a surprise to you.
What I suggest is following a program of training for say a 100 mile ride (in one day). With the wonderful world of bias internet we can find some useful tools like this training schedule:
http://www.angelsofaction.org/wp-co...an-16-week-intermediate-experienced-rider.pdf In order for this to work you have to follow it to the letter beginning with week one even though you said you can do 50 miles in week, but following the schedule to the letter you prevent injuries and burnout. If your goal is not doing a 100 miles in day but say rather 50 is your goal then simply train till week 8. To maintain that level of fitness you will have to constantly do week 8 after that, or if you decide to shoot for a higher level say a few months after you reached the 50 mile goal you simply go to week 9 and continue till you reach your new goal. That schedule is good for base miles, you need to do that first before going for speed, but speed will improve gradually and naturally as you pile on the miles, but to get significantly more speed you will have to do interval training AFTER you get those base miles in. You can come back here later after you completed whatever goal you want to get to and we can go on with how to improve your speed at that point.
Another thing to seriously consider at your age is going to a gym and working out to improve your overall body conditioning, again this will take a lot more time then it did in your 20's but you can improve your body over time. The new thing that science has discovered that working out with lighter weight and doing more reps is just as effective as working out with heavy weights and less reps, but the lighter more reps will be less injurious to your body.
If you decide to go to the gym then I would do the cycling thing every other day and going to the gym on the off cycle days. As you get older you don't want to overwork your muscles or you will become fatigued due to not resting your muscles and that will frustrate you because you can't figure out why you're not improving or feel tired when working out...that's a sign of overdoing it. So don't go crazy at the gym, just spend about a 1/2 of an hour working a circuit working on devices designed to improve your core which in turn will improve your cycling ability.
If you would rather buy your own gym stuff instead of going someplace a machine that I saw that I know works because I know a guy who is 73 years old and uses it every day for about 1/2 hour and since he's done that and walks a couple of miles a day, I've seen a dramatic improvement on his fitness look and level, that machine is the one that Chuck Norris sponsors and uses himself every day called the Total Gym, it's actually cheaper than other types of machines but if you watch Craigslist long enough you can find one for a lot less money.
