This will sound nuts to the average cyclist, but as well as mountain biking I am into weight training and over the past months have added 20lbs of lean body mass through heavy weight training and lots of food, I have only ridden my bike a few times since gaining this weight but boy have I lost my fitness, my question is can I be as fit as I was when I was lighter? is the added weight going to hold me back? If I get out on bike 3 x per week from now on while getting heavier can I be as fit as I was?

you are fit for lifting weights, not fit for bike riding right now. my opinion is that you will have compromise both or choose which one you really want to be fit for.

thanks for your advice mate… love both think I just need to get out on the bike up some nice steep hills at least twice per week, get my lungs used to oxygenating the extra mass I have gained.

if you get back to being as fit as you were, then you'll still be slower than you were purely as a result of dragging an extra 20lbs round with you

Power-to-weight ratio is the key. More power is needed to compensate for the extra weight and that power must be cycling-based.

No point having arms, deltoids and lats like Arnie unless you have Chris Hoy legs to go with it.

ditch XC altogether and switch to DH, 4X and/or Dirtjumping.. best of both worlds 😉