I'll tell you, there's so much mumbo-jumbo surrounding pedaling speed (cadence), it's hard to separate the wheat from the chaff.

The basic thing to understand is, that for any given gear, pedal faster and you'll go faster.

Now, if going faster doesn't matter to you, cadence doesn't really matter; stop thinking about it. The OP asked about "solid exercise," but that's not really a cadence issue. One can work out in a variety of ways, so what's "solid" depends on plan and goal, not pedaling speed.

If going faster does matter, there's one goal (to turn a bigger gear faster) and one question overarching all others: how hard can I afford to work before I'm exhausted.

Within that framework, you adjust your gearing and pedaling speed to accommodate fitness, terrain, and duration of cycling, so that you can get the most speed out of your available effort level.

A power meter is the easiest way to understand all this, as one quantifies your effort into a visible number. So, through experience riding with a power meter, you might learn, let's say, you can sustain 200 watts easily, all day. Knowing that, you pick the gears for the terrain that allows you to maintain that effort level.

Of course in reality, most don't really ride like that, and instead we surge with power, pull back and recover, and do it repeatedly. It's in this case where cadence takes on added dimension, as with experience you also learn how it impacts heart rate, and you'll find choosing a particular cadence range can speed your recovery (lowering heart rate).

To illustrate this, I myself know that pedaling between 85rpm and 95rpm is about the same in terms of my heart rate response, but whereas I can easily kick up my cadence into the 130rpm range (and beyond), it causes my heartrate to skyrocket; add in pedal load (i.e. power output) and I'm not going to last long at those levels. So even if I am in a low power recovery mode, I'm not going to crank faster than 100rpm because my HR just won't drop (or alt least not for awhile). Instead, I'll roll back to 80rpm and my "all day" power level, optimizing speed and recovery both.

You will probably find that, at times such as hill climbing, you won't have the gearing to stay in your ideal power range/cadence combo, and you'll have to pedal slower and let your HR rise in order to keep going. It it what is, but it's the good cyclist who can work effectively across a wide range of cadences with abroad power band, max out the efforts, and have the fitness and savvy to recover and do it again.

So pedaling speed is really about goals, but also knowing how to manage your body and your efforts. I'm repeating myself here, but this is so important: a good cyclist can pedal effectively across a wide cadence range because they have the power and conditioning to do it.