That sounds like a really positive goal, interesting Steve Smythe said I needed to work on my speed, as I stepped off the track completing a 19:32 5K
The good thing about speed work it tends to be shorter and quicker although may hurt more at the time you potentially recover quicker.
I am currently doing a moring run of a mile with the dog which has a liitle incline but a nice gentle slop back.
We have 3 x 5 milers in the summer down here which are high octane runs which all the track runners come out for and the youngsters who are not allowed to distance and they are so fast.
I really enjoyed the 100m slow jog followed by the 400m at pace for the 5K, know there are lots of combinations it is getting a programme that will meet your needs
Of course improved speed over 5K will translate across all distances as you say through to marathon
Looking at races for me locally we have, now do not laugh the
1st Jan Brown Willy Run - good hang over cure running across the moor from Jamaica Inn
13 Jan Westward League Cross Country - very muddy
20 Jan Stormforce 10m - can be very windy and hail etc
Just as a few ideas but really need to get my speed back!