I don't think people should be worried about things being too hard or them being too slow - there will always be others willing to help drag you out of a ditch, push you over a step or pick you up when you get stuck - The main reason why it is good to go trail riding in small groups.
One of the fun parts of trail riding is having a go at doing stuff that you wouldn't normally encounter on the roads, riding through 2ft deep bogs with very little forward motion covering anyone behind you in a shower of stinky black mud (sorry

) as one example.
The best trail rides I have been on are the ones where you stop after each couple of lanes, have a chat, wait for the others to catch up and have a laugh about all the divvy things you have just done. Gives everyone a breather (trust me, most people have off-days and in these circumstances a gentle pootle can be more rewarding than a time-card enduro).
If you want to build confidence up, then send me a pm - I often go to Salisbury plain on non-firing weekends with a couple of others and it is a good place to gain experience quite quickly. I have been riding for a good number of years on the trails and wouldnt really say my standard was any different from the majority of the others, new comers or people with all the training courses. (except Horse's though!).