Mtb Advice
 

I bought a 2009 Trek Fuel ex8 at the end of 2009 and rode it for around a year mostly on pavement and a few trails before becoming a roadie. The fuel has seen minimal use since 2010. It is stock slx/xt, fox suspension, 3x9, qr axles etc.....
This summer I started commuting 2 miles to work on the fuel and would like to start mountain biking more often.

I live in NW Indiana/Chicagoland area so i think the full suspension is a little overkill for around here...

Are there any quick economical upgrades i can make to bring this bike more up to date? (I am not sure what mtb components are compatible with what i have as far as frame, hubs, bb etc. as my knowledge is mostly road stuff.)
Or should i look for a ti or steel 26" hardtail frame to sawp the stuff to, or just N+1 and add a Hardtail 29er to the stable and possibly sell the fuel?

The tentative 2018 plan is to run a few less half marathons next year and do some 100 mile mtb races like the Lumberjack100.

What would you do paceliners?!

I would ride the fine bike I (you) already own. "It's not about the bike." :-)

I'd ride it as-is before spending money on anything beyond setting up the tires tubeless.

What would I do?
I'm not really acquainted with Northern races like the Lumberjack 100. Your Trek Fuel is a fine bike, but if your races tend more towards cross country and less technical descending, a hardtail would save you a little energy in a 100 miler. I can't help but feel that a 29er would roll along a little better on faster courses, but I haven't ridden a 26" in six or seven years.
I have only done races on 29". My go to bike is a Trek Top Fuel, when you lock the fork and shock, it rolls and climbs really well. At my age a full suspension doesn't beat me up as much after 6 hours on the bike.
BUT, I have done Fools Gold 50 on both the Top Fuel and a Cannondale Flash hardtail, and it seemed to be a wash. The full suspension was only faster on one of the big descents. The lighter hardtail certainly didn't wear me out on the climbs and felt much faster on the gravelly finishes. Horses for courses.

I had an ex8 from 2008. Barrying some problems with a chainstay weld, warrantied, it was a fantastic bike.
I went to a fully rigid 26" because I thought it would be simpler to maintain, but I still think about the trek--it was a blast to ride on challenging terrain/ descents and made my work a lot easier.
Take it to some trails and enjoy it.

If you're looking for upgrade to modernize your bike a bit, consider ditching the Shimano 3x9 system, and replace it with 1x set up. You can keep your crankset and replace it with a 32T 1x (Narrow-wide) chainring for about $45. Just figure out what your crank arms' BCD is, then get a 1x narrow-wide chainring to match. Then you can replace the current shifter/rear derailleur with a Sram GX 11 speed shifter and derailleur, and replace the cassette with a Sunrace 11x46T cassette.

Another, simple upgrade is replacing your standard seatpost with a dropper post. Simpler still, consider getting some wider bars with a slightly shorter stem.

I'd keep the triple, a 1x is not worth the hassle of removing gearing options.

As mentioned before, tubeless is great for MTB.

Listen to OfCounsel

The short bar/wide stem combo is a vast improvement, puts you neutral on the bike and it handles better. Go tubeless and ditch the triple as there is no advantage...unless you have a thing for redundant gears and broken chains. The cheapest SRAM NX11 will blow your current drivetrain away, and you can use your current wheel

Ride it like you stole it to see if this is going to be your thing. If you start blowing thru parts, consider a bike change before rebuilding a shock, fork or wheels for that thing. Keep in mind that the 10 years since your bike was new is a few generations of tech that your missing and it isn't worth the $ to fix, not if really like to ride

PS: roadies are suckers for 27.5 cross country bikes...don't do it

PPS: Have fun on the trails

I'd just ride it, the EX8 as mentioned is a fine bike. I had a 2010 that I sold a couple years back as it wasn't ever being used. When it was in use it was a super fun ride.

If your bike uses 26" wheels then I'd suggest you make inexpensive updates or none. If you want to ride more get a new bike.

Your bike is fine, but I wouldn't put any money into it other than to maintain it.

If you are interested in upgrading, I would look at a 29er. I think you will enjoy the benefits, especially for marathon mtb racing.

I would sell the Fuel and get a 29er.

Probably too late for this year, but companies have demo days a few days a year at Palos. Your bike will do fine there, and on demo days you can try something different. I try the latest thing but end up sticking with my old Zaskar.

I would pump up the tires and go ride Palos. Lots of new trails there. You have a great tool for the job.

Don't do anything to your Trek until you've had the opportunity to demo a few bikes and see what the modern bikes can do. Demo both hardtail and full squish, try multiple wheel sizes and widths (27.5+ is FUN!), and then you can start asking yourself what the shortcomings are in your current bike and what you'd like to see in it or in a new bike.