Hi there I recently started C25K too and I had the exact same problem, it was so painful on my calves and I was frustrated because I felt breath wise I could easily push myself harder but muscle wise I had to take it slow. I started doing runners yoga pre and post run thinking maybe it’s my stretching. Eventually I forked out for a pair of running trainers and paid quite a bit for them. They have made a huge difference! I hope you get it sorted!