Do hill sprints once per week and/or lift, including legs, and/or do plyometrics. Do striders a few times per week. For the most part, just gradually increase your mileage so you are running peak mileage at the end of summer.
Summer of Malmo is a solid program, and he’s not advocating 70 mpw or an any mpw target. He’s simply saying run more. And the best way to do this is by doubling as many days as possible and organizing a couple of relaxed workouts per week with other running friends to keep engaged in the process. If you are willing to commit to this, I don’t think there’s a better way to get better. But you have to be willing to do it.
I’m not recommending you do this, but what I did in high school for summer base that worked extremely well was an 8 mile run every day M-F and a 12-13 miler on Saturday. Sunday was off, though sometimes I lifted. Sunday was not the ideal day to lift though, lol.
Anyway, I ran one fartlek per week like on Wednesday and my long run always ended up being a progression run with the last mile or two close to tempo pace. Most of my runs were on a dirt canal or grassy, undulating golf courses, which might be the best option for you if you have any nearby. I just ran as I felt, sometimes fast, sometimes slow. I just let it come to me. Had I to do it over again, I would’ve added a four mile double every weekday. I also think I would’ve lifted less and done short hill sprints and striders more.
Good luck. If you commit to getting better and do take a down week or two, you will improve. You might surprise yourself. But more than anything, you should enjoy the process. 👍