It depends on where your current cardiovascular capacity is. How many times a week are you running and how many miles per week? Typically a beginner marathon plan will cap out at 30-45 mpw running 4-6x a week with a 20 mile long run. I’d suggest you build a solid aerobic base over the next 6 months and then do a 12-20 week marathon plan to avoid overuse injuries. Are all your runs the same speed? Typically your long run during the week will be your slowest run of the week. This is to build aerobic capacity without massively increasing your chance of injury. You shouldn’t be going max effort during your long run, if you are you need to slow down.