Sometimes the weather is too nasty to run a workout on the track but you feel like your training is going to slip without an interval workout. Assuming you already have your aerobic base built, the good news is missing a single workout because of weather or life issues which suddenly arise is not going to destroy your fitness. The bad news is many runners feel the need to tack on a second workout, or do one the next day at a higher intensity than they originally planned.
Listen very carefully to me. Don’t do it. A day without a workout is a day without a workout. It’s not a character failure or the first sign of the apocalypse. Just do the next day’s planned workout & be happy for the extra rest you probably needed in the first place.
If you have access to a good treadmill on those days when the weather is nasty or dark once you get away from work, or you know you’re going to miss a track session because of a social/work function, here’s a good 60-minute substitute workout. I've used this on several occasions, especially in the late winter, when almost NOBODY wants to be out in the dark, wind & cold hammering out repeats on the track:
Start the treadmill on the manual program with a flat elevation; to simulate the resistance of road running, you can set the elevation for 0.5 to 1.0 percent. Set the speed for a pace with which you can comfortably run for an hour; I like to use about 0.5 of a mph slower than my long run pace (for example, if you’re running 8:00 miles (7.5mph) on your long run, set the treadmill speed to 7.0-to-7.1mph. A 9-minute miler (6.7mph) would set the TM to 6.2 or 6.3mph).
Warm-up (15-20 min) – 2.0 miles at starting speed.
First set (10 min) – increase speed 0.2 mph – run 30 sec, rest 30 sec x 8.
Second set (10 min) – increase speed by 0.2 mph – run 1 min, rest 30 sec x 5.
Third set (10min) – increase speed by 0.2 mph – run 2 min, rest 1 min x 3.
Cool down – decrease speed by 0.5 mph, run for rest of hour.
I’ve done variations of this workout (.1 mi/25 sec, .18 mi/30 sec, .22 mi/40 sec & .25 mi/60 sec repeats), but 30-second & minute time intervals are much more simple, especially if you’re math-challenged to begin with.
This is just a simple, 80-percent solution to take care of business when either Mother Nature or Father Time (Constraints) places undue (negative) influence upon that well-laid-out training plan of yours.