how long till back to fitness?

hi guys, i'm now back into mtbing after a 3 yrs absence, in this 3 yrs i have gained 3stone in weight! I am on a strict diet and have lost 1.5 stone. I bought a mtb 3 weeks ago and have been out quite a few times each time returning exhausted with burning muscles after pushing it a bit harder each time,my question is how long does it take till riding gets any easier I seem to have forgotten!!!!!
thanks
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