for runners
While I love running and its my main sport, its high impact nature means that pure running training opens you up to overuse injuries. Since I dislike the cold, and running in it, I've visited the indoor pool and rode my bike into the city a couple of times this winter. I now believe I can achieve 80% or more of the results of pure running training by including the two other major endurance sports and training like a triathlete.
Within a five zone model of intensity, I've found that running at zone 2 is the most pleasant, with zone 1 needing a conscious effort to slow down and zone 3+ needing a conscious effort to speed up.
Riding the bike at anything above zone 1 requires a good course without traffic lights or intersections. Biking for two hours running various errands in the city or just exploring can be almost as beneficial as a one hour long run while being a lot more pleasant in suboptimal conditions. Also, provided you don't live in the US, Canada or Australia, commuting to work by bike is a time efficient and popular option to get a bit of extra exercise in.
I'm sure with perfect technique you can swim in zone 2, but for me a 400 meter swim gets into zone 3 after a lap or two and ends in zone 4. And that's for breast stroke and back stroke. If I were to use front crawl, my heart rate would surely creep even higher.
Replacing the tempo run with a pool session gets rid of a big chunk of injury risk and gives the legs a bit of a break. Replacing the long run with a bike tour keeps most of the metabolic benefit, while allowing for a more scenic route and a longer ride in total.
Since almost no running races are scheduled in summer, but all the triathlons happen there, training for a triathlon also has a scheduling advantage. It fills that gap in the calendar, so there is something to be excited for in the summer as well. As for distance, a super sprint triathlon consisting of a 400m swim, 10k bike and 2.5k run should be well within my reach by this summer.