as a 2 sport D3 guy, i don't know what "walk on" means for D3. no one is scholarship which is what walk on is usually juxtaposed with -- ie, no scholarship. now, depending on the school, they may have tryouts or competition to be entered in meets. but other schools may take everyone, no cuts. depending which one you're at, that would affect whether your plan makes any sense. if there is no "cut" then i am not in the walkon's conundrum of feeling the need to show up peak speed and fitness just to survive a cut so i can get any meets the rest of the year -- even if that means i've peaked too early. if you don't need to worry about making the team, then you should really be preparing for the season and its demands as opposed to preparing for roster battles day 1.
that leads me to point 2. have you actually talked to the coach? i say this because someone who's spoken to the coach usually has a specific training plan, has had the XC conversation, and at minimum "knows the score" on how their track team operates. i knew only so much about college soccer and TF but i knew the soccer team had a cut, that i was expected to make it and compete for a starting job, etc. i was sent a training plan for the summer, a game schedule for the fall, some tactical x and o stuff.
i say this because someone talking about walking on and like there is 9 months of nothing until then, asking the internet for plans.......doesn't sound like they went through recruitment, brought themselves to the coach's attention, got on the "mailing list," spent a couple hours on the phone with coach, etc. if you are in fact admitted to the D3, and signing up for classes and dorms and such, and in some vague way thinking about getting ready.........but haven't talked to the coach.......that's odd. they will be excited to have another solid runner around. they will be eager to pass on plans. they will probably suggest XC which will cover 95% of what you're fussing about, and if not, they will have a plan to bridge you to the spring.