I did a great north-ish trail this year, from hebden bridge to tarbet Ness and back to Inverness for the train. Eleven days, it went like this:
1.Hebden to hawes 92k 1858m
2.Hawes to dufton 58k 1085m
3.Dufton to kielder 114k 2113m
4.Kielder to glentress 95k 1275m
5.Glentress to Stirling 116k 1314m
6.Stirling to crianlarich 89k 893m
7.Crianlarich to fort William 78 k 1788m
8.Fort William to cannich 101k 1343m
9.Cannich to Evanton 95km 1517m
10.Evanton to tarbat Ness, nigg ferry, black isle and back 131km 1051m
11.Evanton to Inverness 63km 818 m
Couple of short days and no big ones, although I found dufton to kielder hard.
I’d probably want four more days to comfortably do the start to hebden and the bit from garbat to Furness. The GNT would be easier than my route in places, and harder in others. Last week of April first few days of may, hardly saw a midge, one half day of rain, below freezing on 6 nights. I’d happily do it later and cope with midge, but wouldn’t do it earlier in deference to the chance of poor weather. Loved it. Planning on a Scottish c2c2c this year.
Edit I’d echo what boblo says, first few days, specifically the Pennine bridleway, are as hard as anything else I encountered.