Hi, Muzza2. Looks like you had a tough time out there but glad it hasn't put you off. Was that the 110K route you were attempting? That's a very serious distance in mountain country.
Sounds like you are building your distances up far too quickly, in which case you are risking getting injured and possibly a little demoralised. I think a more steady approach might be called for!
For training you need to be outside and the best way is probably to enter trail events starting with shorter distances such as marathon or 30-35 e.g. Sussex Coast (Endurancelife Mar 2016?) might be a local one or you need to be prepared to travel.
Don't know about gym work but it is good to have a strong core so I recommend Pilates.
That injury sounds like an inflamed tendon on the top of your foot (extensor tendonitis). Usually due to overuse or sudden increase in running (especially uphill) but can be due simply to having you shoes tied up too tightly. Ice and rest, or very low stress activity (with no uphills!) A useful stretch is heel raises off a step (but not heel drops).
I'd avoid the treadmill, especially on an incline - likely to make this injury worse.
Talking about brutal I did the Midnight Mountain Marathon on Saturday evening. I rank it the second toughest of the ones I've done (Exmoor Coastal being the toughest).
The first half (?!) involved ascents of Craig y Fan Ddu, Fan y Big, Cribyn, and the big one Pen y Fan. Freezing wind and clag on the top at about 2100.
It was all downhill from then on in more ways than one.
I was complimented afterwards on the speed I was doing over rough ground down the Roman Road but once in the forestry on the way back to Talybont it was very hard to keep going. I think it was only the compulsory glowsticks on runners ahead that gave me something to aim at and kept me moving forwards with alternate speed hiking and shuffling. Avoided the midnight cutoff by 29 minutes, for a 6:01 finish.
How the winner managed to get round in 3:17 defies belief.