Is Base fitness for bike and run the same?

Hi all, need some advice here from those who had managed to straddle marathon and triathlon training.

I’m 9 weeks out from a full marathon race in December, and 29 weeks away from a half Ironman race in 2020. At the moment, I am 3 weeks into the Half Ironman Base, planning to go into Build and Speciality subsequently.

However my marathon training is taking a toll on me, with the long runs (now at 27km) taking a longer time to recover. It is becoming clear that I have to put bike training aside if I plan on getting quality run workouts for a PB in marathon.

If I were to start my TR plan only after my marathon, I have only 20 weeks. Do I:

  1. Complete 16 weeks of Base and Build, and then finish the 2 weeks of Speciality before going to taper for 2 weeks, or
  2. Complete the last 4 weeks of Base, and then finish 16 weeks of Build and Specialty.

Essentially what I’m asking is, if my base fitness from marathon training equates to base bike fitness?

Thank you!

In my experience (going other way, being keen cyclist for several years before starting running) the answer is ‘kind of’!

Aerobically yes I was fit from cycling which allowed me to push hard at running and get good times…BUT my muscles were not used to the impact and motion of running and often sore and injury prone until built up ‘running base’.

I imagine it would work similarly the other way around, maybe not as bad since cycling easier on the body. However I know few runners who are reasonably strong at running…but pretty useless on bike so who knows!

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Basically yes as you are building aerobic fitness. Lydiard is the god father here and had it pretty much nailed. I looked for a reasonably short description of his method and think the piece at this link is decent:

This describes the cornerstones of “base” which is Fartlek and long tempo along with some suggestions of how to implement.

Converting this to cycling, SST is tempo. Call it lower SST or 80% FTP (which is a proxy for MLSS). Fartlek is harder to describe but it’s a tempo ride with some short efforts in the 95-105% zone. As an estimate. I’d say just go for a nice ride, turn the legs over, but not a hammer fest.

A big difference between running and cycling is recovery. Running just beats you up more because of the impact and the upper body drive.

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One thing to keep in mind is that the start of base is actually fairly easy. That could be a good thing, allowing you to ease off for a couple of weeks, before getting stuck in again. At the same time, it obviously gets you back on the bike and is progressive, more than just a break. It depends whether you need a bit of a break after your last training block. If you’re feeling strong and keen to get started, you could jump straight into base 2 otherwise.


My experience is that it transfers pretty well. A few years back I did a marathon in May followed by a 100 mile road race in early August. Similar experience to you, in the early stages of marathon training I could combine work on the bike, but as the mileage and long run ramped up I had to cut back on the cycling in order to give my legs time to recover. I did try to incorporate one high quality ride per week, this was before I had a PM or TR, my go to workout at the time was 2 x 20 minutes at around threshold (probably high SS with hindsight). I managed this about 2 weeks out of every 3, I think it did a lot to maintain my cycling fitness. I also sometimes used cycling for active recovery, it’s better than running due to being non impact. Found that my cycling fitness came back very quickly and was back to close to my best within the 3 months.

That said, I would probably still err towards option 1, or even take it further and do 10 weeks of Base then 8 weeks of Build and skip specialty altogether (also not convinced you need a full 2 week taper for 70.3, I find 7-10 days is about right, though everybody is different). 70.3 doesn’t have particularly specialised needs, you need to build threshold and endurance, and I think Base and Build are the best plans to do that. Specialty is a nice to have.

(I don’t think the fitness transfers anywhere near as well in the other direction, since run base is as much about conditioning your muscles, ligaments and tendons to deal with the pounding as it is about building the aerobic engine).


I just listened to a recent Purple Patch podcast where Matt talked about a triathlete that wanted to do a full, stand alone marathon. They kept the other training going. But instead of the very long runs, they did multiples. For example: 1x Friday, 2x Sat, 1x Sun. (one of which was a brick session). This got the total mileage for the long run completed, but without the intense fatigue a single run would induce.

I have over simplified things – I suggest you listen to it. The episode was Post-Season Case studies. Might give you some ideas.


This pretty much hits it on the head. As someone who has a huge running base, coming to cycling aerobically was “easy” but the muscular fatigue took a bit of time to adjust to. Similarly, going between the two, it’s more the muscular soreness and fatigue than the aerobic system. In that sense, you could probably be pretty dang fit for running and do occasional bike work to keep your legs accustomed to it.

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