Poor girl. I am so sorry for your loss. 
My boy was like this when we lost our older girl, he was 7 at the time, had never been an only dog and was really bonded to our girl. He started howling while I was on the school run as well. I was only out for 10 minutes each time, as we live opposite the school, but we still ended up with a neighbour complaint.
I can recommend trying Zylkene, for anxiety, it’s over the counter from pet shops and is made from a type of natural milk protein. I wish I had known about it at the time. My vet recommended it to us when one of my dogs developed separation anxiety and we are now using it again for our elderly dog who has some cognitive decline related anxiety. (I buy mine from petprescription.co.uk.)
We found more interesting, rather than more/longer, walks plus introducing all sorts of brain games, snuffle mats, licki mats, kongs, puzzles, kong wobbler, treat dispensing ball etc really helped wear him out and reduce the anxiety. Essentially wearing him out mentally, rather than physically.
We gradually made progress and he was able to manage an hour alone after a couple of months, but ultimately we ended up getting another dog, which was great for him and he immediately calmed down, but unfortunately for us the new pup (rescue with a very bad start) then developed separation anxiety, so it’s not always the right move.
We lost the younger dog during the first lockdown. He died at home with our old boy there and this time he accepted it straight away and hasn’t had any issues, although sadly he went on to develop a bit of anxiety over a year later as a result of cognitive decline (he’s 16).
Definitely get veterinary advice, but ultimately I think it’s likely to be a case of time, love and patience, wearing her out mentally to reduce stress and perhaps trying something to help manage the anxiety.