Hayward,
I just noticed that you started this type (weighted/boots) of training 3 weeks ago. I don't know what your training background is prior to this, but I would like to stress the importance of both making gradual changes and starting slow. Much like a transition to barefoot running, but slower. My apologies in advance if you're already versed in this.
Both weighted running and walking can be beneficial to your training program, but it is important to realize that training with weight stresses the tendons/ligaments in your feet/calves/knees differently and with more stress than regular training. I'm not talking about muscular/cardio fitness, but the actual strength and ability of these tendons and ligaments. You can have a 2.5hr marathon time & be used to hiking some steep backcountry terrain with a heavy pack, but neither will have prepared your body for walking/running with weight on concrete for any extended period.
When doing your training, I recommend trying to do your weighted marches on concrete, as this is the same terrain you will be using on basic. The most important thing to strengthen with this training is your arches; stop if you feel pain. The beginning of mild planters fasciitis typically feels like a sharp pain from about the beginning of your big toe, along the inside of your foot, to about halfway into your arch. This is caused by an inflammatory response by your body, trying to heal your arch. Stop, let heal, and switch to something else for awhile. Don't stop and run the risk of it developing into something worse (collapsed arches, plantar fasciosis, heel spurs, all nasty).
Running with weight is quite different than walking with weight. Here, the strain will likely be on your knees. Similar to above. Work it in slowly, stop if you feel any pain & switch to something else for awhile. It is very possible to run with weight without side effects, but it takes time.
You also talked about footwear. If you decide to go with runners (I would), stay away from "barefoot" runners for this training. Especially on concrete, your feet need the cushion and more importantly the arch support, provided by a good set of runners. Salomon, at the other end of the spectrum, tends to make very arch-supportive footwear (can actually be painfully excessive for some). There is a happy balance in the middle.
Most modern runners have what looks like an "extra" eyelet a little ways back (towards your heel) on either side, past the top eyelet. Modern shoes are designed with these for a reason -- they give the shoe it's intended shape and helps to stop over/under pronation. It's really a no-brainer to use them, even if you don't tend to pronate without weight. If you're unsure about pronation, a surefire way to see your is to take off your shoes immediately after a hard training session while they're fresh off your feet. Look at them -- Are they obviously "bent" inwards or outwards? If so, take some photos and change up the shoe.
I honestly don't know enough about training with boots on to comment, but I would look for something relatively lightweight and (again) with good arch support. If you find your shoes/boots without the lack the support you need, consider adding an insole like "superfeet" (sold at MEC), which incorporates rigid plastic heel cup to give you more.
Just wanted to mention that strengthening this part of your body is just as important as the cardiovascular/muscular system, and only becomes more important when you add weight & concrete. It's my belief that this is why the CF had so many injury problems with getting the new guys to ruck run.
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Edit: Damnit. Hope this helps someone anyways :\
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