Not @Bagsy but seeing as @dagill2 nicely tagged me in I figured I would add my 0.02c worth. Copied this back into your log so its easier for you to find it if you need to and to keep the conversation here. Hopefully bagsy will comment here too.
Firstly wanted to say sorry for not finding your log sooner and commenting on the awesome work you are doing. Now I have found it I will be be sure to keep up to date with your journey.
OK so lets talk about running. I noticed you commented recently that you got itchy after running. I just wanted to say this used to happen to me to, mostly on my stomach. I liked to think it was the fat burning off that was causing it. LOL
Firstly your question about when to add speed work. For me I would wait a little and re-build my base before any speed work. Remember that your cardio system and your muscles will adapt to the running quite quickly but your connective tissue wont. It takes a little longer for tendons and ligaments to strengthen so I would not try to push too fast too soon. Now that being said if you have a big base of running from your triathlon days and have kept up the strength work since stopping the running then it isn’t going to be as much of an issue for you.
For me when getting back to running I would focus on building my base first, and this is done with the most important run of the week - ‘The long slow run’. I would have 1 run each week where I ran at an easy pace. A pace where I could just about hold a conversation with someone as I was running. I would build this run in terms of time to a minimum of 60 mins, and eventually getting up to 2 hours plus when I was training for a longer run.
If I was running 3 days a week, 1 run would be long and slow and at the start the other two would be short and steady. So the slightly faster tempo but not an all out effort.
Once I got to a point where my long run was over 60 mins then I would add some speed work. It’s worth mentioning that my fastest running was always done when I was focusing on my long slow runs and building my engine.
For speed work I liked to use either a tempo run which could be either tempo intervals or a straight run. Example straight run would be 400m warm up, then run at desired tempo for the next 5km, 1km slower run to finish. I would gradually increase the pace of this run.
For intervals I would either do some work on the track or use my watch to measure out a set distance and do repeats. Track work was simple efforts, 20 x 200, 10 x 400, 5 x 800, 4 x 1000 (note the total distance was usually 4km). Efforts are timed and the goal of the workout is to make every effort the same time. Obviously the pace for the efforts would vary based on the length. 200’s were faster than 800’s
If I was using my watch to to intervals it was usually just 1km repeats, so warm up for 1km, then run a fast km, rest the repeat for 3 more efforts. These efforts were always run faster than my goal 5km time pace. For example if you wanted to run 5km in 20 mins (4 min km’s) I would work up to 1km efforts at 3:30 - 3:45 pace.
I always found that shorter faster track worked helped with my efficiency and technique, where as longer efforts 1-2km helped with my speed endurance. You obviously need both to be fast over 5km.
Hope this is helpful.