If you’re training for a spring marathon then this is an excellent opportunity to practice what you’ll do on race day. So you can wear the same race kit, the same shoes and practice the same fuelling strategy in this race. In terms of your pacing it’s likely the best option is to treat this as a steady long run. If you try to hit goal pace for 20 miles, less than a month from your marathon, then diminishing returns set in. You’ll need too long to recover and it can have an adverse effect on your training over the next week or so. If your marathon is still 6-8 weeks away then adding in some goal paced miles is definitely worth considering.