In reply to The Potato:
I cycle a lot more than I run and found similar issues when I try to increase my running milage, for me it's shorted/tight hamstrings and weak glutes from cycling, sore calf and Achilles, basically needed to do a lot of stretching/yoga and strength work especially for the glutes. I try to go a short 2-3 km run after every cycle so I am to try and keep running legs activated.
Although I love fell running often leaves my body battered if I am poorly trained, so revert back to just flat running which is much easier on joints before trying to build back up the off road stuff.
Not sure you need to change technique, just maybe more stretch and strength work, and maybe different shoes, low drop shoes for example definitely put more stress on calf's, and not sure the trend for super soft high stack shoes is particularly good, especially when used off road.