In a bit of a fug today. Eventually got my act together for an off-road run. Two runs really as I stopped at the churchyard where my grandparents are buried. Slow and a struggle. Back and legs feel sore. Switched my watch off and forgot to switch it on again. Roughly 1hr30 for 11.5miles. Been eating too many biscuits.
First off you're doing very well. 3+ miles is an ambitious start, but it's a good challenge. Extensive stretching wont necessarily help the fatigue and pain you're getting in you calf. Try a tubular bandage to give a bit of compression over the calf muscle during and after the run. But don't push it if you feel the muscle tightening. On the plus side if the strain is going into your calves that might indicate you're naturally a midfoot to forefoot runner which is generally a good thing. Look forward to hearing about your speedy GNR time next year.