I always try to drive or cycle a course I've not done before, and in the case of Tonbridge I've also run the whole course once and bits of it of 4 other occasions.
Its really not at all bad IMHO; a good course. It certainly "undulates" but is no where near as bad as Dartford, Maidstone or Tun Wells - no way! In fact for me its an ideal course with lots of fast downhill stretches that just happen to come after some uphill bits. In all honesty, most of the up hill stretches are pretty gradual inclines. The worst one is a spike into and on the the return out of Liegh (at about 3.5 / 9.25 miles; steeper on the return. but it only goes on for less than 0.2 miles. This will be my wife's first half after a hip replacement and she's certainly not been bothered by the hills.
I don't understand the complaining about 'undulations'. All the best runs (in my view) have some challenge in them. Reading has the run up to the town before the run back downhill to home, Bristol, Hastings, Great North, Tun Wells, Maidstone, Henley, Dartford, Thanet; the Bexhill, Buewater and Darenth 10ks; and as for Bewl 15!... they all have hills which make the race challenging, interesting and fun.
Brighton is pretty flat, but don't be fooled as there's a long gentle haul uphill from about 1.5 to 3.3 miles; its not the elevation gained but the fact it goes on and on. If you want a really flat (and boring) half do the Bexhill one in November. Fantastic organisation but the race is basically four flat laps of the seafront.
I'm really looking forward to Tonbridge. What does a PB matter if you enjoy yourself? Frankly I think a large part of getting a PB is in your mind. And I think this course has the potential.
But Frazer, if you've only run 4 times since Dartford, whilst I'd not drop out, I'd certainly take it easy. Go out steady, a bit slower than normal, and just enjoy it. If you try for a PB you'll almost certainly disappoint yourself, but if you start out steady and find you've got oomf left for the downhill stertches particularly in the second half I think you might surprise yourself.