This is all assuming you want to do an XC race, and not a downhill:
The Gorrick Spring Series are a nice safe bet, and hopefully not too far away. Races tend to be anywhere between 1-2hrs, depending on the number of laps, with lots of fun singletrack to keep you entertained, while you race the people around you.
As you’re over 40, the ‘veteran’ will work if you want three laps. It’s reasonably quick, but not as silly as the Super Vets class. However, for a very first race, why not try the Fun category. It’s only two laps, and there are a real mix of abilities and ages, so you will find people of a similar level to race against. After that one, you will know your lap times and can work out how you’d do in other categories.
The Gorrick Enduro in May and the Big Dog in August are brilliant events, but are much longer courses and you race for longer. Fun as part of a team, possibly quite tough on your own for a first race. However, as they are a few months away, the Gorrick’s mentioned above should probably be your first choice.
There is also a race nr West Drayton (Black Park XC) in March which is fun and has a fast but flat course.