Santa,
these practice days are fairly controlled events. the guy or girl coming up behind is duty bound to do it safely...its not a race. Most of these days will have the standing order of no overtaking on corners, and only on straights when safe or waved by. If you can safely drive on the road you should be OK on the track. Its a skill all race drivers need anyway to check mirrors, midway between corners to check what's approaching. (or hopefully how far youve pulled away from the competition)
As your a novice when you see them coming plan ahead let them take the appropriate line, let them know your intention wave them through etc.
Be aware look around but dont concentrate solely on others otherwise youll fall off or not enjoy it. At a tune or practice day, nobody should be seting serious times in your group, so dont worry about getting in their way. In my experience Some days are broken into CAMS licence holders and non licence holders sessions so you should be out with similar experienced people.

Just do it!

Cheers,

JP