Definitely maintain speedwork during the winter. Don't need to keep high mileage, in fact it's probably healthier not to - it does depend how your body responds to different kinds of training load. If you already have a decent mileage base, just try adding one or two sweetspot sessions per week and take note of any impact it has on your longer distance stuff - are you feeling stronger after a few weeks or is it having a negative impact? If it's the latter, try with one session per week, if it's the former, cool, keep at it.
Definitely maintain speedwork during the winter. Don't need to keep high mileage, in fact it's probably healthier not to - it does depend how your body responds to different kinds of training load. If you already have a decent mileage base, just try adding one or two sweetspot sessions per week and take note of any impact it has on your longer distance stuff - are you feeling stronger after a few weeks or is it having a negative impact? If it's the latter, try with one session per week, if it's the former, cool, keep at it.