I feel your pain. I’m currently trying not to overtrain for a duathlon this weekend. this mornings times were slower than last week, so tomorrows training will consist of walking my dogs. 3 hours of mountain biking probably burns around 3000 calories. The average American consumes 2600 per day. I try to eat like a bodybuilder, start and end the day with egg white omelets. I get most carbs from oats, sweet potatoes, and rice. Fats from nuts and fish. I eat 6 to 8 times a day to get the calories I need for my training, be it cardio or in the gym. I use supplements like vitamins and protein and Omega 3s too.
I read a little about marathon training. Those guys do one long (18 mile) run per week and 3 or 4 days of 6 to 8 miles to perpare for the big event. Three hour rides might be too much. Maybe 2 1 and a half hour rides and 1 4 hour?
I’m no pro trainer, but I have hurt myself over training before.