And then there's places to stay at night.
Camping: good as long as you can get to a site. If you're taking busses and trains and such, however, you're more likely to get dropped off in the middle of cities, the furthest distance from the more affordable campsites... Which brings me to my second point. Camping, unless you're plugging in an RV, sharing a tent site with friends, or otherwise overtly desirous of those wide open spaces, generally doesn't beat a mid-city hostel for value. At $30-35 a night I'd prefer to get a free breakfast and a shower. Hell, when travelling alone I'd probably even rather share a room with a bunch of subsonic snorers than have that isolated isolation of setting up tent and then thinking, "now what"?
Also, if you can plan to never camp, then you don't need a tent or sleeping bag. I used mine once and could've been traveling much lighter if I'd never allowed for the possibility that i might end up at the side of some road for the night (the only time this happened I ended up staying all night at a denny's instead anyways... so there's always that.)

There's this problem with being a tourist who wants to get the insider's perspective on a location: you have to be really intentional about a strategy in which you are not just surrounded by other travelers and un-initiated outsiders. On busses and trains the people most aimiable to strike up conversation with you are likely not the commuters but the adventurers like you- looking and ready for novelty. Unique also to this walking approach is the fact that many Americans- i dare guess the majority- don't really USE sidewalks. They use cars. Those on the sidewalks are going to be the odd ones out. Same goes for those on the city streets in the middle of the afternoon, when most people are in office buildings, industrial sectors, or storefronts actually working their 9-5. Misfits hang around on the street in the mid- afternoon, so your experience is skewed, overpopulated by people like you: transients, oddballs, and misfits.

Hostels naturally attract the likes of European students in a gap year- who are nice but forgettable (if i'm honest in a general sort of way). Other characters include people who are between houses, cities, or jobs, or who are simply on a business trip and too cheap for a hotel or who experiencing some kind of mid-life re-discovery and are self-defined nomads. Sufficed to say: not necessarily "Normal" folk, and unfortunately much less interesting than they often seem to think they are. Again Ruthless generalization, but there it is.

That said, i would use hostels again in certain situations. I think they are great for a certain kind of traveling and really are supurb value- often staffed with very helpful people who can show you how to traverse the local neighbourhood.

But what do I reccomend? Or perhaps better put, what would I do next time?
Probably don't presume to walk around America. Its not that there aren't great places to walk, but these are places, in themselves, that you're probably best getting to by car. America has been defined, probably more than any other country, by highways and the automobile industry.
I'd also look for ways to get the inside view on a destination, especially an urban one, which is to say I'd try and stay with people I know whenever possible. I never tried Air bnb or couchsurfing, but either might be good ideas, but require planning ahead.

More planning all around would be a good idea. Not that this wasn't adventurous. Some flexibility is good, especially when given the friendliness of people you may be directed off the beaten path to worthwhile side-spectacles. But this doesn't mean you can't have a general intinerary.

Finally I'd prefer to travel with another person or people next time. Yes, this limits flexibility to some extent, but it also works the other direction when it means many costs are split (campsites, hotels, gas etc.) as well as duties (navigation, communication, driving). Its nice to have company and if you get hungry, you can always revert to cannibalism and eat them.