I'm going to provide some input on this because I'm currently a walk-on at P5 FBS program that finished in the top 25 this past season. I am also a civilian looking to commission upon graduation, and I am currently looking at the different officer accession programs. Walk-on is a VERY ambiguous term as far as roster spot/recruitment. As the posters mentioned above, walking-on isn't as easy as it may seem, and certainly not as glamourous as the movies put it. Simply put, a majority of walkons spend their first few years a glamorous tackling dummies, helping run the scout cards and help out on drills for the guys that actually play on Saturdays. A few end up seeing the field on special teams and garbage time, and every now and then one will crack the starting lineup.
Walking-on means different things in different places. For some places (like my school), all the walk-ons are preferred walk-ons (PWOs) that all had scholarships offers elsewhere (including other P5, Group of 5,FCS, D2) but chose to play for their some state. Three of the PWO guys in my class have started in games, and two are now on scholarship. The PWOs here are treated the same as the scholarship players (free meals, training room, tutoring, locker room, clothes etc etc), aside from the fact that we have to pay tuition.
Other schools have a more "traditional" walk-on program, where they invite tons of really good HS players to summer school and weed out who really wants to be there. No promises for making the 110 man fall camp roster, but you might get invited back onto the team when classes start. A lot of these kinds of schools have varying levels of treatment for their walk sons, such as a separate locker room, not having access to the dining facility, and less clothing/swag/gear. Sometimes your name won't even be on the roster.
All schools have walk-on tryouts, but it depends on where your at if you actually have a shot. I think (I could be wrong) that all programs are required to have tryouts, it's just some don't advertise them (and you actually have to reach out to the staff to find out when they are), and it's more of a thing to check a box. I know one guy that made it to the team from a tryout and he is no longer on the roster. I am certain anyone would have a better shot making team as a tryout walk-on at a school like VMI than it is at Alabama. Both are D1, but there is definitely a talent gap between these teams.
Regardless, if you end up making a D1 squad, forget about the possibility of doing ROTC. I know there is the NCAA 20 hour limit, but it's really a 40+ hour weekly commitment when you account for all the "optional" things in addition to showing up early, treatment, meals, and travel. It is certainly worth the experience, D1 football is certainly not up to its hype as a player (especially for a majority of walk-ons). The football team owns you for about half the day during the week, all day Friday and Saturday (if you make the travel squad) and all summer, so attending summer training is impossible unless you want to jeopardize your camp spot.
Let me know if you have any questions. I was not an All-State/Region/League player in HS, but I certainly played my cards right during the recruitment process.