Nimrandir,
I know exactly what you mean. I have been there and I was there with this lesson too though I could get rid of it within the week (sort of). It comes with practice really long and tiring practice. I have up 16/17 years ago when I was in 2nd/3rd year of self learning. I tried a lot, watched every video I could grab of (Petrruccis being one which helped a lot in AP). When I joined GMC 3 years ago, I was pretty much in bad shape. I could hardly do 16notes at 120bpm. Like you said, I too suffered from "no stamina", both arms hurting pretty badly. Things have changed a little now. I can do some speedy stuff and my arms know quite a bit of stamina. When it hurts, I like it coz everytime it hurts, I know in next 2 days, I can do it.
Back 2.5 years ago, I was in MTP (very similar to this program) and I practiced few speedy licks given by the instructor. Add to that, regular REC stuff, challenging myself harder lessons every time, I was able to overcome the pain problem and it just got easier than before.
Few things I have learnt -
Practice (I practiced even if it pained a little, I didn't over do it so I was injured),
hard work ( I practiced the same lesson like 3 months before posting a REC which wasn't perfect still, I felt bored but I had no choice. If I have to overcome this speed thing, I have to do it),
Patience (3 months or even 6 months on the same lesson can lost your patience, I took a month break and tried other lessons in between but then coming back to same one),
realise, I realized what mistakes I am making, such as picking pattern. If I am concentrating on one lick, I start with down stroke but somehow when I played the whole thing, I was using upstroke hence the whole lick got screwed, left hand fingering, hand sync (I still notice my mind is slow with the left hand so I have to start few millisecond ahead of my right hand.
research, I do a lot of looking around in YouTube just to get some inspiration or if anyone had similar problem like mine. Most of the time I land on Guthrie's videos and man they are just amazing.
Anyway, I am sure you know a lot, I am just sharing my experience. I gave up on this lesson too but after a day break I started again and there I was doing 150 next day.
A week is too short for new things. I have practiced 2nps before so this one came out easy.
Keep practicing!
Chandra