how do I get started training in the winter?

Max heart rate:

www.brianmac.co.uk/maxhr.htm

www.topendsports.com/fitness/karvonen-formula.htm

As you look at all the different formulas for max HR, it sounds confusing. But if you calculate them all, you will see for most people the final numbers fall into a relatively narrow range. I put all the formulas into a spreadsheet and average them. I figure that's close enough.

if you use the Karvonen method to fine tune the target range, you will notice it changes as you gain fitness and your resting HR changes.

You should not be looking for a diet and exercise regimen to do for a few months to get in shape for next summer. Ideally, you are seeking a lifestyle change to stay healthy forever. So you need to make changes you can live with and not give up in a few weeks. The best diet and exercise plan is worthless if you do not stick with it. (Which is why Michelle Obama's healthy school lunch initiative sputtered -- kids didn't want to eat it.) If running outside in winter creates problems, then don't run outdoors in winter. You also need buy-in from your spouse and family.

If you are willing to spend a few hours per week on the computer, there are free on-line university courses available in nutrition, health, and exercise physiology ranging from basic to grad level cellular biochemistry of nutrition. Free, unless you want academic credit. Browse the catalogs for courses that tickle your interest. (There's a course in Shakespeare from Kings College in London that has my eye. If I am going to take a course in Shakespeare it will be from a UK university! )

Coursera.org
EDX.org
FutureLearn.com (UK based)
Open2Study.com (Australia based)

Coursera has a current course on the science of exercise from Univ of Colorado, started about 2 weeks ago. You can still enroll and catch up. It is a bit basic. Watch some video lectures, do the assigned readings, a couple of easy assignments, on line forum discussions.

An easy method is the sing-talk test.

Workouts can be broadly classified as easy, moderate, and vigorous.
If you can sing out loud during the workout you are in the easy category.
If you can no longer sing but still carry on a conversation someone it is moderate.
No longer easily converse with someone other than grunts and monosyllables then you are in the vigorous zone.

I like what Walkswithblackflies has to say---just get out there and go. Winter's nice, summer's nice---both are Altars in Miss Nature's cathedral.

There's no point in overthinking all this---the best training for backpacking is to load up a pack and hike, preferably up a mountain. The more backpacking trips you do the better shape you'll be in for . . . uh . . . backpacking. I like Walk's pic and here's one of mine---

I've been doing a lot of hill-repeat workouts outdoors at night lately. I always try to wear my reflective vest, too, but once in a while I forget.