With Railmiles, you can indeed add previous journeys, as long as you have the departure location, date and time, and the arrival time. You would add all of the details into the "new journey" section. The website then finds your journey and adds some extra details like the operator, the length of journey, which trains (if information about this is available*). Once you press save, it'll be added to your journey total (you can edit it in the future if you need to.
* Not all operators show their unit allocations, such as Merseyrail and West Midlands Railway.

[ A screenshot from the Railmiles websites "New Journey" page, with boxes for the origin and destination information that need to be entered.]
Once you've added a journey, in the menu if you select "Public Site", you can see a range of information, such as distance travelled, most recent journeys and most frequent journeys. If you go into the menu again on the public site, you'll be given options to see all journeys, and some statistics about the trains you've been on.

[ A screenshot from the public page if Railmiles showing information about the number of miles travelled, the most recent journeys, the most common journeys and the longest journeys]

[A screenshot from the journeys page of Railmiles, showing information including the date, origin, destination, traction, distance, headcode (ID), times, delays and speed for 8 journeys]
Railmiles is a paid for subscription, but it can make tracking your journeys easier. On the journeys page you can search for journeys that you've made, either by date, train (or train class) or operator but annoyingly (unless I can't see it) there isn't an option to directly search by station (although you could use Control + F, if on a laptop/computer, and find the stations that way)