I find it easy to mix the two, and it opens up more competition opportunities. I time trial every week throughout the summer, enter duathlons on top of running races, mountain bike through the Brecon Beacons. Just adds to the variety without the impact of running and I don't agree you have to train 3 to 4 times as long on a bike compared to running. An hour at a heart rate of 150bpm is 1 hour regardless.
Duathlons are mostly summer but there are some winter ones. Duathlons are normally run under British Triathlon Assocation, so a good place to look is the BTA website, although their 2006 events schedule is not out yet. Another place to look is www.triathletes-uk.org.
The winter events I know about are off-road for both running and cycling, like the gears and tears monsterman series.
I run and ride and my current schedule, due to it being winter and dark morning/nights is Mon - Fri am (before work) run approx 45mins
Mon - Fri lunch time cycle 45mins from work
Mon - Fri evening cycle 45mins from home.
Sat, either long run (1 - 2hrs) and short cylcle (1hr), long cycle (3 - 5hrs), and short run (45mins), or brick session - cycle then run off the bike.
Sun - Rest day.
The lunch time cycles are generally quality sessions, e.g. tempo, interval, etc. Whereas the evening sessions are steady sessions. Living in the country without street lights I don't want to cycle too quickly into a ditch.
Running during the week is a mixture of intervals, fartlek and hills, with easy recovery runs.
OOK, it depends what you want to achieve. If you run for fitness and pleasure then your approch is fine, if you want to run competetively then a more "scientific" approach is needed, i don't think Paula would have achieved what she has by "just" running.