Do you have access to a calf press machine? Maybe, you could try to hold for ten seconds or so between each rep, repeating for a few sets. Remember to not bend at the knees though.
Leg presses are good to increase strength in the legs generally, but to focus on the calves and not the hamstrings as much, you need to avoid being too close; go too close and it will just work the upper hamstrings. Make sure you don't lock the knees and use your toes to get a full press extension. Maybe also try one leg at a time, when doing presses, but remember to work both legs equally.
Just a few other things though. Maybe you could try a 5km cycle on 'non-leg days', followed by a few 1km sprint-cycles. It will work the hamstrings, but should build up the leg generally.
Final question, are you getting enough protein in order to put on muscle?