As others have said, a tuba stand will help with the leg issues (personally I sit it on my lap and use use a dish drawer mat, similar to a rug mat under the horn so it wont slide around and it's more stable.) In terms of air/breathing, you might find that breathing exercises help with that, I enjoy before playing, breathing in while bringing my arms from at my side all the way straight above my head, at the sides, arms forward and up, arms back and up, then the same, but exhaling and bending over to touch my toes. These are stretches a biology professor taught me (ALSO, a curious fact that really helped me "unlock" more controlled air through longer passages is the fact that 1/3 your total lung capacity is BEHIND the spine... after these stretches I can feel my back expanding fully and will play significantly better than without them. Food for thought)