First class

[VenusdeVelo]

bikeless,
As far as your training, I do think you will find your time on the spinner (assuming you are doing 3-5 classes per week, 45min - 1.5hours each) will get to you your goal of being able to do the 100km. Indoor training is often considered to be more concentrated and efficient than outdoor training (no downhills, you can't stop pedaling etc.) -- Chris Carmichael (Lance's old coach) used to quote that indoor training was as much as a 10% increase over outdoor training. The number's disputed (of course) but something to consider that you are typically getting in an efficient workout.

The issue though which could crop up is without outdoor training or a spinning marathon (many clubs offer this in advance of century training), you are not spending a concerted time in the saddle, the amount of time you will spend when you do the metric century. So let's assume 15mi/hour average, you'll finish your ride in 4-ish hours. Sometimes it's a big surprise to people who have never spent that focused time in the saddle. If you can find some ability to go on say a 40 mile ride outdoors or find a 3-4 hour spinning event, it would provide you with good preparation. All the time you spend seated though will probably help you with this because that's usually the hardest part -- keeping that butt planted for that long! Outdoor riding will feel easier because you just natually have more moving around on the bike and less stress on your errr..."backsides". ;-) (You didn't say if you've ridden this distance before so wear shorts with good padding and use chamois cream for extra comfort!!) The periodic standing and such will all help, as you mentioned to help you shift the pressure.

If you are able to maintain consistent cardio while spinning, your training should be good prep for your ride as well on this front. A word of advice though -- once you get the HRM (pulse clock) fixed, watch your HR throughout the entire spinning session. Staying at 160 (which as you said is likely close to your max - simple method would say your max at age 56 is 164 but we can estimate with better methods) for the majority of your spinning session will burn you out on an endurance ride. It also is not necessarily providing you the most aerobic training benefit during your spinning sessions. For max aerobic benefit, you should maintain your HR in zones 2,3,4 and only train up near your max when you are focusing on some interval trainng (maybe 1-2 times a week thrown in to your spinning sessions). Zones 2,3,4 will be in the ranges of the following example, can change depending on whose training you follow, the example below is based on a max heart rate of 177:
Zone 1 55% 65% 97 115 DISTANCE
2 66% 75% 117 133 ENDURANCE 2
3 76% 80% 135 142 ENDURANCE 3
4 81% 90% 143 159 INTERVAL-HILL-PACE
5 91% 100% 161 177 PEAK

The great thing about spinning classes is you control your heart rate, you can work with adjusting tension and cadence according to where you want your heart rate to be. You don't have as much control outdoors, so it's an optimal training tool. If you can get your hands on Bicycling magazine's issues where they cover training for a century ride, or see if the info is publicly available on their site (I think it's www.bicyclingmagazine.com), this was a good source I think to put things in perspective.

Hope it helps, sorry I am repeating stuff you already know ;-) Best of luck on your training and your ride, I am sure you will have a great time! Keep us posted!

[VW Beetle]

Yeah, a lot of the guys over at Bike Forums were pooh-poohing spin classes, but I find more credible the ones who seem to be serious cyclists and admitted they got WHUPPED in their early spin classes.

It seems to me that with a HRM you inevitably would get back what you put into it, so I can't see how one would NOT improve significantly with spinning (subject to the time-in-the-saddle caveats that VdeV notes)

[bcipam]

I wish more clubs would offer longer spin classes. I'm usually just getting warmed up after an hour. My old club used to have back to back classes so I would stay and do both. Now I usually (if the class is open or not being used) go in 30 minutes before to warm up and then stay another 30 - 60 minutes after class. Got some good tunes on the iPod and I'm good to go!

I do think spin is tough, at least a good class is. But it is different than riding where you also have to contend with gravity and road feel, friction and wind. Both are good cross-training tools for each other. After an injury I was unable to ride on the road but was able to do spin and whent he fractured healed I was able to do a century without much loss in fitness.

[traveller_62]

After reading through this thread again I have to say that I have a much better appreciation for the challenges that spin instructors face in designing their classes.

The diversity of needs and expectations on the part of those of us who take spin classes is pretty amazing! It is fortunate that the nature of the spin bike allows people to adjust a workout to suit their needs.

My hat (and bike helmet) is off to those of you who teach the classes and try to make them interesting and enjoyable!