It's possible you're experiencing a set back from the walking. 1+ miles a day is a lot if you're in some kind of fatigue state.
I agree with the others on doing aggressive resting.
The vast majority of people with postviral fatigue recover, so even if it's taking a long time, the odds are in your favor. So my suggestion would be to hang onto the thought that you're going to be one of the ones who do recover.
I think the main things to work on now would be getting good sleep and alleviating your anxiety and depression.
My suggestion would be to start with gentler non-aggressive interventions first - just in case you do have ME/CFS it's easy to make yourself worse because you may be sensitive to meds and supplements. Then if those don't help, try something stronger, but try a low dose first to test your reaction. Then if it's tolerated, you can try increasing the dose.
So my overall suggestion would be magnesium which is calming, and may help with sleep and anxiety. It's usually well tolerated. Any form except magnesium oxide, which doesn't absorb well. So something like, magnesium citrate, taurate, glycinate, etc.
The rule of thumb for magnesium is to gradually increase the dose over a number of days until you get diarrhea, then back off a bit, and that is "your" dose.
An Epsom salt bath would do the same thing (that's magnesium sulfate).
Then talking to a professional psychologist (cognitive behavioral therapist) can be very helpful. I had one while I was getting sick with ME/CFS (I had a gradual onset). She diagnosed me with "adjustment disorder" - basically it means you're experiencing anxiety and depression because life has thrown you for a loop. This is a natural reaction to what is happening to you.
She helped me deal with the uncertainty of not knowing what was going on, if I would ever get better, finding a doctor who could diagnose me, and eventually when it became evident I was going to be disabled, applying for disability and finding a disability lawyer. Therapists often have connections to doctors and lawyers they can suggest, so that was very helpful.
A therapist can help you learn various relaxation techniques such as progressive relaxation or meditation and help you get out of the thought spiral that causes you to have more anxiety.
If you can't get out of the house, there are therapists at DoctorOnDemand.com, who you can talk to via Skype.
If you have more than mild to moderate situational anxiety or depression, they will let you know it's time for medication.
There are some alternatives to SSRI medication, such as the inositol Hip mentioned. 5htp is another option. They both increase serotonin. PharmaGABA is another - it raises GABA, which is calming.
It might not be a bad idea to see if you have adrenal fatigue. You can have both sleep issues and anxiety from that. You can do a 24 hour cortisol saliva test for that. You can get a combination test that tests both your neurotransmitter metabolites and adrenals called a neuroadrenal test.
This is the one I got last time -
http://www.integrativepsychiatry.net/neuroadrenal_profile.html. The nurse practitioner there was very nice to talk to and had some good suggestions for me for both neurotransmitters and adrenals.
Based on my experience, I would suggest trying things like that first, before trying an SSRI.