With Hadleigh Country Park on our doorstep plus a network of trails through the local woods we have surprisingly good off-road options. Family-friendly routes include the sea-wall from Leigh Station to South Benfleet Station (perhaps stopping off for a pint at The Gladys) before continuing around Canvey Island. It is possible to cycle all the way around the island although the section from Hole Haven to South Benfleet Station can be a little muddy. Another enjoyable ride can be had by taking the train to Limehouse and cycling the Lea Valley past the Olympic Park and the White-Water Rafting Centre near the M25 all the way to Ware.
Langdon Hills Counry Park is a good option for the next level, with Thorndon Park (Brentwood) and Epping Forest providing tougher trails that are only a short car ride away.
The ultimate is of course the Olympic Site at Hadleigh Country Park which has a variety of trails from family-friendly to full-on Olympic cross-country technicality. The Hadleigh Mountain Bike Club is the best point of contact for further information.
Details of mountain bike racing can be found at the Mud, Sweat and Gears web site - they have a series of races at various venues in Essex and Suffolk.