Agree with most of whats been said there, sportsman class is what i nornally enter or E2 clubman. An ACU license is £25 for your first year then £42 each year after, that covers trials, moto-x, enduro and something else. Far cheaper than £10 per event, wouldn't bother with power up parts to start with just get used to being among other bikes and finishing 3 hours first.Almost any event will do.
You can learn the art of time keeping enduros just put yourself in the beginners or Sportsman class, most clubs will have one.
Or if you want to do a multi lap event, such as a Hare & Hounds, which is usually a shorter more flowing course with no stoppers, but 2 hours or so duration, and you don't mind being passed by flat out MXers.
You usually have to enter beforehand for most events and you will need a licence, but you can sometimes get a day licence for the event to try it out.
Long distance trials are fun and you get to ride all day on the highways and byeways to a set route with marked feetup sections, and is a good way of finding out legal trails.
Nothing improves your riding more than competition, the sooner the better.
I have used my 03 XR400 for H&H, enduros, trials, Long Distance Trials motocross, commuting and trail riding. It has very few modifications apart from being lowered, and all the heavyweight headlight kit replaced. Aluminium Handguards and Renthals are a good investment and a good silencer such as a FMF power Core which sheds some weight and improves power, without too much of a noise penalty. Set your suspension 'sag' correctly and the damper clickers, I go all the way in (Stiff) for motocross but standard for trail riding with other stuff somewhere in between. You can get a slightly more free flowing air filter together with a less restrictive cage and file out the internal exhaust header where it meets the cylinder head to get rid of the lumpy weld for slightly improved exhaust flow. I use standard Honda spares for brakes and stuff as they work and last... and second hand castoff enduro road legal tyres from my local enduro expert (Daryl Bolter) that only have one events use.
Keep an eye on your tappets, wheel susp & head bearings, change the oil regularly, don't over rev the engine. Use a 'Venhill Featherlite' cable for a super light clutch. It will reward you with over 30,000 miles of hard use before you have to go inside the engine.
Nothing can touch an XR400 for value for money off roading.