There are plenty of gadgets and apps which help track your movement and activity through a day. My own path to quantifying my exercise was gradual. I started with tracking distance and frequency and later became more about performance than measuring distance.
First a little background. I was never a "runner". When I ran, it was part of another sport, such as soccer. I was a swimmer, so I knew what putting in time doing the same thing over and over meant. I started running after my second son was born and wife suggested we run a 5k.
So when I started running, making it around the block was an accomplishment. I was active, but not cardiovascularly fit. It didn't take long for me to build up on my distance to the point where I could do a loop near my home consistently (still less than 3mi).
It was then that I started tracking my running. Initially, I used the car to measure my distance and tracked time using an Ironman sportwatch. I would track my runs using pencil and paper taped to the inside of the kitchen cabinet. I gradually added other longer routes which I calculated using GMaps-Pedometer. It is a simple useful tool that allows you to map out a route and get the mileage easily. You pick a point and continue to mark points as it tallys up the mileage. I still use this to do route planning when I'm doing longer runs. It's light weight and easy to use.
Seeing that I beat a best time on a route was reinforcing, as was not hitting that mark. Occasionally, I would use a an on-line calculator to determine my actual pace over those miles, though at this point, I didn't have much of a reference for what was "good" for my age and experience. I wasn't worried about pace, but only whether I did better than before.
Pace is great to compare yourself to others, but I needed only to
better myself. I've gradually realized that racing and running more about what is "good" for you regardless of what others are doing.
I continued to track my workouts on paper for a while, but tapered off that as my runs became more consistent and gains were not a dramatic. I started tracking again when I started training for triathlons. I started using a free tool called Map My Run (and later Map My Ride). The benefit of this is that you can track your mileage and workouts in one. You set courses which tell you your mileage, then can log a workout against that course. If you use a GPS enabled device, you can also sync it with this instead of manually logging workouts. Map My Run added another dynamic to my training. It showed me my weekly miles and when I'd worked out (and when I didn't). It was helpful to see the total mile for a week at the crept over 40 or 50 miles.
I later bought the basic Garmin Forerunner (forerunner 10) to see if I liked it. At about $129, it's not a small investment, but not massive either as I'd been racing for about 4 years at this point. This model does pretty much all I want.. It displays pace during the workout, displaying it every mile, total mileage, course using GPS, and can do intervals as well as remind you when you fall under a target pace, neither which I've used (at least not intentionally).
Where the Garmin really comes into it's own, is when it's synced with the computer using their Garmin Connect software. You can then pull up a view of your workout, very similar to Map My Run, with elevations and the course, except that you can view your pace throughout that workout and see where you're falling behind. How fast are your uphills and how fast are your down hills etc. For running, pace is good indicator as I generally don't vary too much, however on the bike it's virtually useless unless I look at two workouts on the same stretch or similar incline/decline, because your speed can vary so much depending on the terrain. One thing I absolutely love about the Garmin and the bike is that I can see my average speed and top speed for the workout. It's always a bit of a game to see if I've broken my top speed (41mph) and how my average was.
I know, there are other tools for monitoring performance such as heart rate monitors (no interest) and power meters for cycling (too expensive for my needs). These are just a few ways to get started quantifying the workouts your doing. Frequency is one measurement, duration another, then there's performance. As I became more involved (not necessarily serious) about training and racing, the performance piece became more important. Decide where you are on the spectrum and get started. The key is to start.