but recon the bag would help with upper body fitness.
not so much upper body fitness - toatl CV fitness - its just bag work is extreme when you kick as well - harder than sprinting which is the sort of anarobic work out you need when you've spent 10 mins full on getting out back and you are knackered and that good ole clean up set comes along - the ability to do a quick 30 sec sprint even though you think you are drained !
just for the record
i did Wado Ryu / kickboxing - got 1st dan
and a year doing JKD(Bruce Lee) Kali (Philipino sticks and knives) and Vale Tudo (cage fighting)
i think i got into surfing because it uses all the martial arts skill but in a bit of a mellower way apart from the big MF wave that beats you up and doesn't care or even notice you, at least in training they ask if you're ok after - my board has hit me in the chin harder than anyone in the ring or training ever did ! - still got a lump on my jaw bone !