It seems odd to me that riding is hard on your legs. Especially if you're already in condition from running and group rides.

When you ride alone do you end up staying in too hard a gear, so that your cadence drops and you mash the pedals?

I have the opposite problem in that when I ride in a group I get into too hard a gear and mash the pedals, but when I'm alone I spin with little effort.

Getting into an easier gear and spinning will also make life better for your achilles. Less force going through the gastroc and soleus, plus the achilles will get the relief when you pull up on the pedals.