There's all kind of things you can do to improve your bike leg, from getting a newer bike to simply training more, but I'm going to focus on just a couple of things I've found to be valuable in preparing athletes for IM races (in particular), which kind of seem opposite to each other but actually work in harmony on race day. Let me explain some more.
In a long bike leg, 90km to 180km, you're going to get tired and pedaling will just feel harder…like you're riding through sand. Even though you might be pushing the same / similar power it doesn't feel like it. You've run out of endurance, so to speak. Something to help your whole bike performance - not just in this situation - is including big-gear riding in training. And quite a bit of it. The intent is to develop your muscular strength and endurance, your neural familiarity with this type of effort, and train your ability to keep pushing the gear when you're legs are fatiguing.
This strength endurance work can be included in a variety of ways, but with the common element of pedaling at about 60-70 rpm, or about 20-25 rpm less than you would naturally choose - we're not training you to be a big gear rider or change your natural cadence.
As with adding any new / different stress to your training, you should build into it gradually. Start with 2-4 x 5 min efforts with 2-3 min easy between. Your target effort is about FTP wattage, or the kind of effort you'd ride in an Olympic distance triathlon. They key is the muscle tension you are developing, not the anaerobic effect - if you're breathing hard then you're not quite doing it right. It will burn your legs more than your heart and lungs. You can do these on flat, uphill or on an indoor trainer…aim for the same effort on the pedals.
Once you're used to 5 min efforts, extend them out to 10-15-20 min up to total of about 30-45 min per session, just once a week at this stage. For variation, do some of the reps standing up…this will burn your quads!! Big gear work can also be included into your long ride, alternating intervals of normal gear/cadence with big gear work, ultimately building up to intervals as long as 60min, completing 4 of these in a long ride - two at normal cadence, two at big gear. The difference with big gear efforts in your long ride is to do them a IM (or slightly above) effort - a power meter is ideal for measuring this effort.
Including big gear work throughout your IM build phase can pay big dividends in the last 40km of the bike leg, however having this new found strength on the bike is not necessarily a strength in itself. It is simply developing an aspect of fitness you will call upon late in the bike leg.
The second aspect of training to help bike performance is learning to "soft pedal" as you ride, which is somewhat contrary to developing big gear strength, but the best thing about having a performance strength is not necessarily using it. In doing an IM bike leg your goal is to expend a minimum amount of effort for the maximum reward / speed, which you can do by "soft pedaling" and barely touching upon the bike strength you've built up.
Saying "soft pedal" needs a very careful definition, which is a subtle change in the way you ride and "how" you generate power through your pedals. It is best illustrated on a power quadrant analysis, where the aim is to shift the power spread from Q I and Q II towards Q III and Q IV. The feel of this just as "soft pedaling" implies - being softer on the pedals, by a small and subtle amount. So rather than "pushing" during the pedal stroke, you would ease off just a fraction, and perhaps even changing down a gear at times. Your power would barely change, but your perceived effort would. As I said, it is subtle and represents a variation in the way you ride…having a smoother pedal stroke.
Learning to soft pedal takes practice, especially during long rides where your effort is most like an IM bike leg. During shorter, harder rides you do need to pedal hard - including during any big gear work - so it is during other gentler, steady-state efforts where practicing soft pedaling is best done. Having a power meter helps a lot, especially for post-ride quadrant analysis, but even without a power meter the feeling of soft pedaling is what you are after. To an extent you are aiming to protect your bike strength - to not use it - even though you eventually will, where your big gear work will come to the fore.
These two aspects of training seem at odds with each other, but are actually complementary. Using big gear work builds your bike strength, especially for late in the bike leg, and soft pedaling protects you from having to use that bike strength until as late as possible. If you get it right - pacing is crucial - you'll get off the bike with fresher legs and ready to run to your potential.
