Robert_Charlton - 7:24 am on Jul 6, 2013 (gmt 0)
Saffron - For everybody's reference, here's the most recent Matt Cutts video on the topic...
Does linking my two sites together violate the quality guidelines?
Matt Cutts - Apr 24, 2013
As Matt says, it makes "perfect sense" to link together a limited number of sites that are "thematically related".
In the example you provide...
...both are about exercise, but riding and jogging are different.
These are certainly thematically related in the context of exercise... and for my taste, the interlink would depend on the context.
I'm probably more conservative than most about interlinking sites I control. I would never link these for the sake of PageRank, and I wouldn't interlink the two sites with global footer links.
If there were a situation where it was appropriate to say something like: "Another exercise site in the SaffronSite Group of sites", and it was something you felt it was helpful to users, then I think it would be fine.
I'd more likely do it on a Company or About page than I would, say, from home. I'm sure others will differ with me on that. I'd also make sure that the sites have established independent backlink profiles before interlinking them.