Constantly Tired Calf muscles

Have you been doing any strength training?  Incorporating recovery weeks?  Alternating easy and hard workouts?  Eating some carbs/protein right after working out?  Recently changed your running form?

I think there could be a lot of factors.  Do any of the above apply to you?

I would start incorporating leg work in your gym workouts at least once a week.  You don't necessarily need to build them up or work them to exhaustion, but you need to make sure that opposing muscles stay in balance to prevent injury.  For instance, if you don't do enough hill running, your quads and hamstrings can get out of balance.  If nothing else, do work on your inner and outer thighs/hip ab- and adductors.  They are not worked out through running and biking and weak adductors can set you up for ITBS (I had quit my leg lifting routine and eventually developed ITBS).  Right now, I do squats, leg extensions, hamstring curls and then leg lifts for inner and outer thighs. 

Definitely, start alternating your workouts so that you don't do two hard workouts on back to back days.

Also, try soaking at least your calves in very cold/ice water right after running for about 10 minutes.  I did that after my long runs and found it to help a lot.

Hope you and your calves feel better!