I'm 56 and I ride 3-5 double's a year, somewhere around 7-8,000 miles a year. Last year I did five double (four of them being quite hard). I ride the AIDS ride every year from SF to LA for the last 14 years. I never really take off during the winter since I'm in a great riding area (SF Bay Area). Since the beginning of the year, there are times when I feel like I'm bonking. No power to the legs and for a while a racing heart. I had it checked out and even had a 24 hour Holter heart rate monitor. The doctors findings were "clinically unremarkable". I made sure I hit some steep hills (which I try not to avoid...ever). Oh, my preferred type of riding is road, almost 90% of the time on my fixie. I don't do doubles on the fixie (except Solvang). I ride 100-150 miles a week on average spread out over 3-4 rides.
So, what I've noticed is that on longer rides, after I've had lunch, I feel 100% better. Is it that simple? When I start out in the morning, I eat the typical oatmeal, hard boiled egg and banana. I have a good diet regimen but admittedly I'm a little overweight at 190 (I'm 5'9").
Any advice would be appreciated.