I can see why you would think “If I can do 16ths at X alternate then just doing downstroke 8ths should be easy”, but alternate picking is a really easy motion and you’re actually having to BREAK the motion to do just the downstrokes.
There’s no substitute on this for just doing long stretches of endurance work, I find particularly going down to a slow tempo where you maybe start to find it hard to control the rhythm and get it sounding really chunky (both on donwstrokes only and alternate).
If it ruins you after a minute, maybe tomorrow you can do a minute and ten.
I’ve mentioned it before but Zero Signal by Fear Factory is a great endurance workout - I like to see how far I can get into it before having to switch my wrist-based technique to an arm-based technique* out of fatigue - 6 months ago I couldn’t even make it to the first chorus up to tempo, now I can just about get the whole way through.
*Just to add, I prefer playing from the wrist because I feel like I have a lot more control both over dynamics and speed/rhythm, whereas going for that extra speed boost from the arm feels a bit like “tense it up and hope for the best”