There are two schools of thought, and to be honest, I've tried it both ways. I've tried more quality work with less miles and more miles with less quality. For me, the biggest payoff was building a base with a high volume of miles over a three month period. It's like building a performance racing car - you want to build as big of an engine as you can and then figure out how to fine tune it. You just want to make sure your body can handle a heavy workload; otherwise, you risk injury. And you also want to make sure there are recovery paces built in a couple times a week.
That said, there are some people who swear by fewer miles but doing more quality work. My belief is that a lot of these people are mixing in cross training. Whatever you do, as you build up your workload, make sure you're rolling out after long runs, mix in a strength regimen, and get lots of sleep and stay hydrated. I don't know what your current running schedule / volume is like these days, but I can't stress enough that you want to do all of the extra things required to stay injury free.
In terms of Boston qualifying, also keep in mind that you'll need a time below your qualifying standard. So while it says 3:10, you really needed close to a 3:02 for the 2021 race. I anticipate it will be closer to normal next year, but you'll still need to run probably closer to a 3:06 to feel confident about getting in.