We did the southern section a few weeks ago on tourers with 25mm tyres on. So long as you have a bit of rubber on the tread and the tyres are in good nick you will be o.k. The tracks make a change from battling cars all the way.
As Mcshroom says there are one or two rough tracks but the only bit that I would not tackle again was the last section into Berwick where you actually cross a field and in wet conditions it would be pretty mucky going. My advice would be to stick to tarmac at Sea House and go uphill to join the A1167. This is the Old Great North road, which is quieter now that it's bypassed and it's a good downhill run into B-O-T.
The track through Druridge Bay country park is o.k. if you take it steady. It's much nicer than the road alternative.
You have to make a conscious decision to go to Sugar Sands beyond Boulmer. It is doable with care , may be a bit of walking , but in the past we have done it with kids in child seats so it's not that bad.
We stayed here
http://www.coquetside.com/contact/ on our first night but it is a good hike if you start at Newcastle. It's right on the harbour and they will put bikes in the garage. Ignore the two nights bit, they were happy to relieve us of our cash for one night.
There is plenty of choice in Berwick, from cheap and cheerful to boutique
http://www.granaryguesthouse.co.uk/ .
We ate at Limoncello, an Italian restaurant at the back of the Kings Arms hotel. Cheap, good and very popular.
We went the Borders route and stayed at Innerleithen, just to make a change from coast but whatever takes your fancy. It's not too stupid hilly and there are coffee and lunch stops at Kelso ,St Boswells, Galashiels or Melrose.
We stayed at
http://www.gledeknowe.co.uk/ in Innerleithen and there's a good pub, Traquair Arms, within walking distance for evening meals. Gledeknowe is geared up for cyclists and lets you put bikes in garage.
Approaching Edinburgh the Sustrans route takes you in a long circle round the city to approach from the east but it is mainly traffic free. if you want to get to the city quicker, other bike routes are available.
If you don't want to stay in the city itself , we have stayed out at Priory Lodge, South Queensferry in the past. Good breakfast IIRC.
Buy the Sustrans map and you can't really go wrong.
Once over the Forth , we followed the Fife Coastal path , but it has more rough tracks than I cared for , especially round Aberdour( ?) where it runs parallel with the railway tracks. We would like to do the C and C Northern route sometime.
I hope the sun shines on you.
