I did this event last year, it was my first 300 too. I really enjoyed it, it was the first audax I did where I was not last or near-last (which should tell you it is pretty easy compared to some others). I will be doing it again this year too!
While it may be classed as shoestring on the Audax UK website, there are good sit down cafés on the way to Skeggy. Warm porridge with cold cream at Gannets (I think?) really hit the spot. Poppies restaurant at Skeggy is a good, large, audax-friendly place too. (nb presumes same route as last year).
It's pretty flat (slightly lumpy bit thru the Lincolnshire wolds, nothing too bad though), and the difficulty of the ride will be determined by the weather - last year we had headwinds and rain all the way to Skeggy, then got blown back to Alfreton.
Any 300 specific tips for a distance newbie? I've only done one 200 before, and that was a close thing on time. "Train more" being the obvious one.
You will need lights, and it might get surprisingly cold at night. Otherwise, pace yourself and, er, try and get some more riding in first - you still have 2 weekends.