PG3 - Sunday morning, the A23 (main road into Brighton) will be mostly closed from Carden Avenue, just before elite Start, so you won't be able to get to central Brighton by that route, if you want to park in a multi storey in town so you aren't too far from finish you can get to Churchill car parks by coming down Dyke Road, past Brighton Station and down West Street BUT the traffic getting out of there during the afternoon is an absolute nightmare!
I would park on street in the area north of Preston Park - around Surrenden Road, you will definitely find space there in under a mile from start and it's free. The 5B bus from the Pavillion will take you back there afterwards. buses.co.uk to check details.
You'll need to look at a map to make sense of the above, don't use sat nav as it probably won't know about road closures, coming down the A23 you need to turn off not later than Carden Avenue, I'd turn off before then because there may be a queue there with people asking what to do!
Last year quite a few people were late to start so you will be able to run even if you are late which with chip timing is OK if you're just looking for a time but not good if you're looking for an age cat place.
Baggage trucks close at 0910 so that's the real deadline to beat.
From Newhaven, London Road Station is closer to start than Brighton, about a third of the distance and 5 mins less on the train so that saves you 10-15 mins.