If you kill infected, there's no telling how long it'll take to level up depending on how high of a level you currently are. So, probably your best bet would be to start New Game+. That's what I did, anyway xD.
You don't have to go into new game + for the hammerfist upgrades. Any of the consume upgrades that you missed will randomly show up as a new consume target in free roam eventually. I failed to get several skill and power upgrades the first time and was able to get them all by the end of the game. Just keep wandering around and you'll eventually find them.
edpikachu posted... So if I start new game+ do I only get the upgrades when I get the hammerfist from the storyline?
Nope, getting the powers doesn't upgrade them in NG+.
It's still possible to find brawlers to upgrade the powers in your older game, but that's more a matter of actually coming across them, since it's random.
In NG+ you'll get a lot more marked enemies that give you upgrades so your best bet is to do NG+ which is fun because you can now insta-kill choppers and tanks from the get-go. --- "It's not about black or white because we're human." - Tupac Shakur