I'm in the same situation as you are right now (will be walking a few weeks behind you) and have settled on a liner/blanket combo. While a sleeping bag may be necessary for the first part of my walk, I don't think I'll need one as my trip stretches into June - whereas a blanket can still come in handy for picnics, etc. even if I don't need it for sleeping. (I suppose one can always use a sleeping bag for that purpose as well, but since blankets are generally lighter than bags that feels like a better option for me.)
I've been a big fan of Kelty products for many years and am considering packing my Bestie Blanket as part of my sleep system. It's not particularly light (just over 1.5 lb.) but it's durable and snuggly.
Finished with super-soft fabrics on both sides, the Kelty Bestie Blanket is great over rough rocks or pokey grass...around the campfire or the park...as an extra layer over or under your sleeping bag...or a gift for your BFF, IRL.
www.kelty.com
Of course, if a credit magically appears in my REI account (stranger things have happened!) I may consider a Rumpl NanoLoft Travel Blanket, though at 52x38" it might be on the small side for you (I'm 5'10" and it would be on the small side for me, but the light weight and small volume would be a worthy tradeoff.)
Trapping warmth inside small pockets of air, the Rumpl NanoLoft Puffy Travel blanket effectively works like down, but is lighter, water-resistant and packable for all sorts of cozy adventures.
www.rei.com
I'm also considering a generic lightweight fleece blanket, which has the advantage of being something I wouldn't mind parting with at some point if I didn't need it anymore (I'd hate to give up my Bestie Blanket - we've had some good times over the years.) Also seems like it could come in handy for the flights and train trips I'll be taking before and after my walk. Easy to wash/dry at high temps too in case a bedbug situation rears its creepy crawly legs.
And any of the above with the Sea to Summit liner I have would keep my entire sleep system well under 2 1/2 lb., which seems reasonable.
Shop our range of cotton, Tencel, fleece, and silk sleeping bag liners, essential for wicking moisture, adding warmth, & keeping your sleeping bag clean. Your next-to-skin layer for extending the temperature range of your camp sleep system.
seatosummitusa.com
Since you're walking a few weeks before I am keeping warm will be more of a concern - but you could likely tweak the liner/blanket combo to find something that works for you and that will still be more versatile (and possibly lighter) than just a sleeping bag. Will be following this thread to see what you decide!