After doing one myself which finished yesterday, I can echo some of these comments!
The trains I ended up taking are on my RailMiles and can be found
here as well
I personally focused on miles, started to take pictures but soon realised I would prefer to get as much of the network in as possible and if I liked the area, I have all summer to come back for a day trip
Didn't do too much Voyager mileage if that's what you wish to avoid, but didn't do much loco hauled myself!
People asked me why I had a plan; after all, I could take any train... but to make the most of the rover you need to really plan it. Personally I picked a few areas you haven't been to before; or wish to visit; then find the cheapest hotels in the area. Make sure you know what time you need to be at the hotels to check in, and what times breakfast is/check out is likely to be. Don't be tempted to skimp on sleep; we tried that last summer and by the 7th day we "wasted" it by just going home as we were so tired! Obviously you don't have to stick 100% to the plan (i.e. I skipped the 380 from Edinburgh and did the 67 loco hauled instead; also took East Coast Grantham - Peterborough instead of the EMT 158 as I guessed it would be packed; and it was!) but it will avoid you just missing a connection; or missing an interesting working.
Personally I would go for a midweek start; learning from last year I started on a Monday and ended on Sunday; the lack of services on a Sunday made it a bit more difficult but this year as it was in the middle this year I used the day as a 'day of rest' and enjoyed the later start, to avoid wearing myself out and doing a journey I knew would take a while! (Heart of Wales)
My last tip is to make sure you get reservations for peace of mind, especially if going with someone else. I didn't really end up using all of the ones I had, but it was worth the peace of mind. If you don't want to bother a station Scotrail Telesales are more than happy to issue them; they will even have your name on them
Above all, plan it for fun and flexibility. If you can handle an 8am start and a 10pm finish every day go for it; just don't be surprised if you get worn out after too long!