Don't be nervous. If you've got a decent bike and a reasonable level of fitness (you don't have to be fast just comfortable sitting on a bike for a long time) you'll be fine.
When you see what other people are riding on, you'll be amazed. I saw one lady on a hardtail mtb (with reasonably slick tyres admittedly) but with a child's seat
and child on the back
I know it's pretty flat but that to me was pretty extreme
There are all sorts of old clunkers and BSOs you're surprised are up to lasting 300km irrespective of their riders. And then there are the usual 'all the gear and no idea' brigade on flashy bikes who walk at the first (and every other) hill
Organisation is absolutely first class although the were some odd food choices - it's probably worth carrying some stuff you're happy with just in case you get to a stop and there's nothing you fancy. But one oddity I enjoyed was 'bilberry soup' (translation by my Swedish friend) which was sort of warm thickened Ribena. Sounds revolting I know but actually it was nicely fruity, not too sweet and the starchiness made it much easier on my stomach than straight fruit juice which I find way too acidic (same with most energy drinks). And anything warm was nice until the temperatures got up after midday (we had a fairly cold and rainy night and morning).
Good luck - I'm sure you'll enjoy it