Well I guess I'm the exception here. I believe that "seat" time is at the top of the list for improved riding and "gym" time is way, way down. I know 280-300 pound guys who can ride their ass off all day and "fit" gym types who are crushed physically after only a few miles of technical riding. Getting in shape can help you to get riding better a little bit quicker but never swap "seat" time for "gym" time. Trail riding uses muscles that even very fit people don't commonly use so the best physical training for riding is actual riding. I do believe that watching training videos can be very helpful. Proper technique and line selection can make your riding much less physically demanding. Trust us it will get easier with more seat time.

We think a like. I am all for being in shape, but I am not anymore, I left that crap in the Army.
But working muscles are different than gym muscles, seen it time and time again. I may be fat, but can hold my own in a days worth of work and then still want to play after the work is done.

Once I get the bike fixed I plan to make a trip down to your neck of the woods!

You are fortunate that you approached this forum early. You have a chance to learn to ride with Good technique rather than have to unlearn bad habits. There are loads of YouTube vids and DVDs (Gary semics, Shane watts etc) that will help you understand the correct position. You need to practice the correct positions to the extent that they become your goto "oh sh!t" position. Standing is one of the most important skills to learn. I was fortunate that I started on an '82 xr200 with terrible suspension so if I didn't stand the bike would kick me off at any opportunity - this is most likely why GG decided not to put any cushioning in their seats, they don't want you to use them ;-)

While I agree fitness is very important I think I have learnt more being unfit :) When I had my first child all else stopped but I stubbornly continued racing tough enduros and I quickly learned that if I was going to survive the laps I would need to make all climbs in one go without having to return to the bottom and do over or fall and have to pick the bike up. It quickly became 110% concentration to recall and enforce everything I had read and heard about dirt biking and taking a minute to select good lines. With fitness I would most likely have bulldoged my way through everything. Now that I have proven to myself that the theories work in practice fitness will help me do it quicker and more often