Lots of walks available it seems! However, I got to the following post only before I felt the need to comment:
1. Exercise
2. Killing time before catching the Fort William overnight
3. Popped in a couple of pubs on the way
4. Saved the tube fare!
Good call! Kensington Olympia to Euston isn't that far anyway is it? Not done it myself but I'd say it's what, 4 miles at the most. I could do that in less than an hour...
Over the years I've done plenty of walks in London to save money. In the mid-2000s mostly! Paddington to Kings Cross and vice-versa was the usual choice, and back then I was definitely not physically fit, but I could still do that in 50 minutes. These days, with ultra-powerful legs I could probably do it in 40 minutes allowing for pedestrian crossings, and I won't lie to you I want to find that out now. I'm in London tomorrow as well, hmm...
Last April, following a late-notice appointment with BA, I got back to London earlier than expected. Now while I fail to note what exactly I did journey wise, I had a lot of time to fill before a coach home. I did a fair bit of walking in London (by now my fitness levels were much better, having nearly finished losing 5 stone of excess mass at the time) and somehow I ended up at Embankment. Must have been due to wanting to take some touristy photos. I could have easily walked to Victoria pretty quickly, and at my rate of walking I'd have only been a matter of minutes behind an S-stock set, but as the weather was good I did what you did, stopped off at a couple of pubs along the way!
Never ceases to surprise me how easy it is to explore London (and indeed many other cities) by foot. I used to love taking a tram through a city to explore, but these days I prefer a flipping good power-march. Copenhagen is a good one to do by foot, and the airport is easily reached by foot. I think even with a quick visit to a Netto it only took 90 minutes from the city centre, I would need to refer to my trip report for the timing.
Anyway, enough of me trying to promote exploring by foot. My only useful example here would be Warren Street to Euston Square, by LU it would take far longer than a short walk down Euston Road. Certainly a different way to transfer to the sub-surface lines if coming from say Green Park on the Victoria line and needing the north side of the H&C in a hurry. I think I did that walk after a couple of beers at Euston Tap (when I finallly found my last Victoria line sets) and even with beer in me I did platform to platform Warren Street to Euston Square in less than 10 minutes. I'd never done that walk before that occasion either!
Other useful station to station walks I like. Liverpool Lime Street's mainline station to Liverpool Central, in my experience it is WAY quicker to walk it than it is to faff around getting to the low level platforms at Lime Street and waiting for a train to Central. Again, even as a novice at the time and when I was still very unfit I did that in less than 10 minutes.
Oh and there's nothing like a good power march than Birmingham New Street to Birmingham Snow Hill. Call me crazy if you will, but that uphill section is good fun!