I think it's the slack head angle and seat post angle that is catching you out.I had pains in my calfs, hamstrings etc. Legs felt shot. Went from an Ali Giant with a 71' head angle to steel 456. Bikes approx the same weight. Giant faster and easier to pedal on climbs and on the flat. The 456 a bit harder work on the flat and road climbs, but here's the rub. Given the choice I'd pick the inbred anyday, its a lot more comfortable on longer rides, a lot more stable on tricky descents, it flatters your abilities. I have not come off yet! It took about 2 months to increase my fitness to a level where I can now no longer tell any difference.
Stick with it, it gets better, honest.