I do agree on the whole that training solo can be difficult. But it is no where near impossible. I have to train solo, that's just how it works out monetary-wise, as well as solitude being a preference of mine. No, you can't bounce your ideas off of other people. No, you can't spar or have a support system. But, you can get into a solid, focused environment. Just you and the bag. The bag is the biggest baddest fighter you've ever met. You own him. Nobody judges you, just your own focus. Granted you are missing some things you couldn't get in a more standard environment, but you can get much more in other areas.