from the ever-so-brilliant coach Jorge at beginnertriathlete.com :
Tapering is nothing more than reducing training load enough to allow your body to recover while doing enough to sustain the maximum amount of gained fitness. Every time you train you get fitter but you also can get proportionally more fatigue. Think it like this, every time you train you earn fitness and fatigue points. Usually fatigue points worth more than fitness point but fitness points linger longer than fatigue ones. You get more tired after a session but you recover faster and the adaptation gains (fitness) will last longer.
The reasoning behind tapering is that you want to keep as much fitness as you have achieved during months of training but at the same time get rid of as much of the accumulated fatigue. When you taper you get rid of that fatigue but you can also start losing some of your hard earned fitness hence while the volume is diminished we still perform some intensity to generate enough load to ‘maintain’ your fitness while allowing your body to recover at the same time.
With that out of the way; you mainly want to reduce your running volume during tapering while keeping some runs (with some intensity like strides) to recover while maintaining fitness. Since you also bike and swim and if the marathon was an important race for me then *I* would keep some of those swim/bike sessions but I would keep 'em short and easy for the last 10-14 days leading to the race to get a nice aerobic workout without the stress induced by running. Maybe something like 30-45 min swims and max 60 min bike ride.
No one knows you body better than you and tapering is very specific to the person; if reducing your running volume is plenty for you to feel that you are recovering and ready to kick booty at the mary then keep the swim/bike as it is, if you need more rest make swim/bike sessions shorter/easier and go from there. I prefer to lose a tad of fitness but making sure I am 100% recovered and ready for the race.