Until relatively recently I was even lower tech; using a piece of string on the OS map and then measuring it against the scale rule on the bottom (Is this beginning to sound like the four Yorkshiremen sketch?)
If you wanted to update your tech, without buying anything or carrying anything else on your bike, you could use the 'Directions from here' option when you right-click on Google Maps (might be different if you're not using Windows), and click start and end points. You can then right-click again to use the 'Add a destination' to add control points. I have found the distance to be accurate compared to GPS and map measuring, although the height gained I would describe as 'questionable'.
Another free alternative, is https://cycle.travel/map where you can pick your points and the 'Mountain' button will show you your profile. My research indicated this is more accurate than Google maps.
For what it's worth, I only use my Garmin Forerunner 35 for training, and then I have it strapped to the bars to tell me how fast I'm going during intervals. I don't do Strava and chase segments.