...4 other things will too.
Poor road surfaces
All of these meant that at worst, we were averaging 7 to 8 mph where my normal speed is upto 15mph.
So worth taking on view on those too, fair to say the hills in the SW on gravelly lanes are slower than fhe flat lanes in Essex!
How was your state of fitness? Your normal speed was 15mph on the flat, but was that laden?
My training rides will necessarily by hilly, here in Dorset. I'm currently fit for walking (e.g. a recent 30-miler with 5,250 ft of ascent), but that doesn't translate directly into cycling fitness - yesterday's 20-miler has left my thighs tired! I'm also doing numerous exercises aimed at avoiding knee issues.
According to the guy on whose route mine is based, 90% is on tarmac, the rest on packed dirt of grass, e.g. towpaths.
I'd hope that, by going S > N, the prevailing wind will help rather than hinder but of course, in this country, you never know!
I tend to have a huge breakfast then only cereal bar/peanuts/isotonic en route, with maybe a coffee & cake thrown in.
So, I'd hope to average at least 10 mph, ideally 12 mph, but I certainly take your points about the obstacles to that target.
My levels of fitness? I cycled 4k miles this year and was happy doing 70 milers. But, living in Anglia I had very little experience of hills and most of those miles was on good flat roads. So my 15 mph saddle time shrank quickly as our routes were on hilly crappy lanes. Think I would invest loads of time in searching out decent and safe A and B roads if I did it again given just how poor many of the sustrans routes are. Or get locals or Lejoggers advice from people who,know them and can recommend them.
Cant begin to emphasise how poor (gravel, potholes, steep) some were and had it been wet I'd say some of them would be treacherous /unrideable. And I am not overly cautious generally!
Good luck and am sure people on here can help finding a suitable route to suit your needs.