When most people think of interval training — if they think of it at all — they think of an intense regimen reserved for competitive types. Distance runners heading to the track once a week to work on speed (evidenced by the Swedish fartlek approach that came into vogue here in the 1970s), cyclists working 30-second county-line sprints into their 3-hour rides, swimmers who periodically inject a speed lap into their long hours in the pool.