I have not played a note in 19 yrs until Thursday this week

I went along to a local band for a listen and ended up sitting on the back row for 2 hrs with one of the bands cornets

I really enjoyed the experience but only managed a hymn before i lost all feeling in my lips and couldnt play, after a few minutes i started joining in again and so on and so on...i reckon i managed about 4 or 5 minutes at one point but nothing more without my lips collapsing.
Being a Euph/Bari player of many yrs ago, i had never played cornet before, but decided to buy one and try my luck on it. I tried it for the first time at home last night, and although i can make a reasonable noise out of it for a few notes, any more than 2 minutes playing and my lip is dead again!
My range isnt great either, I can play from the low f# up to the top of the c scale (middle c), but even that is an ask at the moment, and above middle c (in the stave) is a 'no go' zone.
I know ive only played the cornet for 3 hrs total so far and so cant expect too much just yet, but what i need is specific advice for getting my stamina back so i can play a 2 hr rehearsal without my lip turning to jelly, and also advice on extending my range upwards. Playing that middle c on a cornet still feels like playing a top c on a Euph!
The only music i have available is the Arban i bought yesterday, so if anyone has some advice on which exercises i need to stick to i would be grateful, I have around an hr each day spare for practice.
If you need it, my cornet is a 85 besson sovereign with a DW2 'gob-iron'.
Thanks!
(I am back at band rehearsal a week next thursday and would love to be able to play straight through with no rests!)