Well, I thought, I would set several preconditions before making the attempt: it would have to be good weather, on a day when I didn't have any meetings outside the office, when I was in good health generally, and I'd have to find a satisfactory way of planning the route. I then realised about the middle of this week that the first three of these conditions were likely to be fulfilled yesterday. After a little more digging I came up with http://www.routeyou.com which actually specialises in calculating bike routes in the Netherlands and Belgium, punched in the two addresses, printed the result, took a deep breath and set off.
The inbound leg went rather well, I thought: about an hour and a half for 23 km, 14.5 miles, the longest single trip I've done on a bike in 20 years or so. I had chosen the "shortest route" option which directed me through pleasant enough commuter villages and the odd woodland path, and culminated with a long and agreeable downhill coast along the avenue de Cortenbergh towards the EU institutions. The worst part was the supposedly state-funded bike paths which are not well maintained. An hour and a half, door to door, is often as long as my commute takes anyway if I am unlucky with my bus and/or train connections. If I could somehow gain access to a shower at work, I'd be inclined to try this a bit more often.
On the way back I chose the option for the "nicer" rather than "shorter" route; 25.5 km instead of 23. This took me along a mostly parallel set of paths and roads, but to the south of my earlier route. I did not like it as much - the morning run was definitely nicer. It started with much less bike-friendly Brussels suburbs (with the odd poorly-maintained bike path), and then - after actually rather a pleasant run alongside the lakes in the park at Tervuren - brought me to a km of narrow trail with huge stinging nettles on the left and a sharp drop into a stream on the right. The next bit appeared to be an overgrown grass and dirt track uphill in the middle of a field; I couldn't quite believe it and backed up to the hard surface to try and get my bearings. That was a mistake and I eventually found myself in Neerijse, much to the south of Korbeek-Dijle where I had hoped to be. The detour added a good 5 km to my journey, which means I passed the 50 km mark somewhere along the way; probably the most I have cycled in one day in 25 years (since a tour of Donegal with my friends from school).
I have a long train journey today and tomorrow; I can see myself insisting on an aisle seat and doing a lot of walking up and down in the carriage as my limbs and especially my backside continue their protest against unaccustomed service. But I may repeat the experience before too long.
(.kmz files of the two routes for Google Earth here and here.)