There are no mountains in Belgium?
You mention you mountain bike in Belgium, and people laugh at you? I tell you, it may not be the Alps, but you can work up a pretty decent fitness level anyway. And for those that keep bugging you, give them this link: What is mountain biking? Please note in particular the part about "... it need not take place on a mountain." ... it used to say this also on the UCI official web site, and I wish they had kept that part when they revised their web site ... as I think it's important to point out.
Staying for some time to come in Belgium?
Then consider signing up for O2 bikers membership ...
Organized rides (nl. toertochten, fr. randonnée)
Belgium being a very bike-crazy country, you can find a multitude of (non-competitive) rides being organized every weekend all over the country. These are the initiative of many bike clubs, but are also a way for youth organizations etc to earn some money. The principle is that the organizers provide for several way-marked loops of varying distance, you pay a 3-to-5-euro fee to ride one of them, and you have one or two rest stops with free snack and drinks out on the trail. At the end you usually have the possibility to wash your bike and take a shower.
These events can be well worth going to. If they are well set up, they allow you to ride areas that normally aren't accessible to the public.
A paper brochure can be picked up for free at most bike shops. Through that very link you can also search for particular rides that you've heard of or ridden previous years! Another very useful calendar on the Internet is www.bikers.be (click on Agenda). Here many times you can also see if a specific ride has been cancelled last-minute. A third alternative of a calendar for these organized rides can be found on (Flemish) www.mountainbike.be (click on Kalendars).
Some of these organized rides also offer inofficial time-trial (chrono, brevet and raid) possibilities, usually for the distances of 50km and up, and hence might attract more speedy riders (or long-distance amateur riders).
Long-distance biking
Looking for long-distance/marathons? Check out in particular www.zoenk.be, www.marathonbiking.nl and www.mtb-marathon.co.uk. Last link in English!
Competition
I started riding in the US, and over there you can race beginner, sports, professional and expert level. That is not how it works in Belgium: you have the fun class, but except for that you seem to have only one racing level, the racing level. I was highly disappointed when I moved my bike over to Belgium, as I think the feeling of winning, whether it is beginner or expert level, should be granted to as many people as possible. It is an empowering feeling, and should not be only for the elite.
For official competing/racing and ranking you would need an UCI-approved licence. The main licensing body in Belgium is kbwb-rlvb. If you have a license and participate in an event where it is not a requirement, the license can give you a better starting position. For kids (7-14 years), check out the Kid's Trophy.
Mountainbiking in Belgium