Bob, just to clarify, did you buy the Explorer LT or the Ultimate Explorer LT Backpack?
My first question - were you properly fitted for the pack according to your torso length? That's the most important criteria in selecting a Backpack. As someone else mentioned, most of the weight should rest on your hips rather than your shoulders. Most Backpacks include "load levelling straps" to allow balancing of the weight between hips and shoulders.
I'm over 50 and also travel with an Eagle Creek Backpack (2003-vintage Continental Journey). I've found that with tighter restrictions on carry-on luggage imposed by the airlines, it's almost certain that the main pack will have to be checked (despite what EC says about being carry-on ready with the Daypack detached). The Daypack works really well for carry-on. Some of the best features of the EC packs are the detachable Daypack, panel load design, stowable harness system, front-carry option for the Daypack and the built-in "rain jacket".
Regarding carry-on luggage when travelling on European airlines, they usually allow one carry-on bag only, especially in the U.K. That doesn't mean one carry-on item plus a purse or laptop bag - it means literally ONE bag only.
Note that if you'll be travelling on any of the budget airlines in Europe (especially RyanAir they have VERY tight baggage limits, so expect to pay a surcharge unless you pack light.
I've found that even though I'm not walking a great distance with the pack, it's important to have proper fit. The trip from the train station to Hotel or Hostel can be some distance and can include walking over cobblestones or multiple stairs, so a Backpack works well. It's also easier getting on & off trains and since packs are a bit compressible, they can be "fit" into overhead racks on trains a bit easier than "hard sided" luggage.
Happy travels!