I am not a barrel racer, but I have heard that Fallon Taylor’s Barrel Racing College app is really great. I think you have to pay for the membership, but I have heard people at other barns shave seconds off of their times.
I'm no great person at barrell racing but I would try some pole bending and maybe set a long line od barrels and go through them , in and out. Try some stamina and agility exercises. Sorry I'm no expert on barrel racing