Most of the filters available focus on biological contaminants because you can filter them out. Dissolved chemicals are tougher, there are some filters that can take care of that, but we are looking at reverse osmosis systems, and those are $$$. Depending on what you need to filter out, mechanical filtration should be enough for the short term, since you will get a lot of the contaminants out of the water simply by removing the sediment. Won't be perfect, but better than nothing.
With farm land you are looking at ag run-off which will certainly have e.coli and other such nasty bugs you will want to filter out. Some waters will be very high in nitrogen, which might cause a blue-green algae bloom, which the filter won't help with. Depending on the number of people moving along the same path, you could have viral contamination as well, hep-A, noro-virus and others. Or if no one else is walking, plenty of farms with wells that might be negotiated with.
The other method for removing dissolved chemicals is charcoal absorption. the only trouble is that you have no real way of knowing when the filter is saturated, or how much its actually removing. There are a lot of factors involved.
A little research should be able to get you water quality on local rivers and ponds, since it is monitored. As well, you should be able to find what sort of disasters might cause that water to become contaminated, and what you would have to do about it. For example, lets say you work near a large plastics manufacturing plant. They have a fire which due to smoke closes a bunch of roads. The toxins in the smoke may contaminate the rivers, but really you aren't walking out of the smoke either. in that case you would be evacuated upwind. and that may, or may not be in the direction of home. Say its a tanker spill, traffic gets messed up for half a day, then you head out.
If you are talking total societal breakdown, not sure what to say, other than water might be the least of your worries. But any plan you make needs to start with more than just "walking home" why would you have to go? why would you not just catch a grayhound? how much danger is 100 miles of walking going to put you in, rather than staying put and trusting that your supplies at home will get everyone by?
As for filter selection, to actually answer the question. Decide on how much you want to spend, and then pick the best of breed in that range. A pump filter would likely be better for you than a gravity one, as I'm assuming mobility is your primary goal. I prefer the MSR miniworks for a few reasons. Its field maintainable, its easy to visually check the integrity of the ceramic filter. It has a decent enough capacity to handle the water needs for several people in a reasonable amount of time. the inlet tube allows you to get water from pretty well anywhere you could reach, and it threads on to any standard nalgene widemouth.
There are others, the sawyers seem very popular, not my pick for a couple of reasons, they seem fiddly to get water for more than one or two people. Their marketing hype really turns me off, it just feels over-sold, and If the water had some sort of solvent contamination, there is a risk it could compromise the filter membranes. But they are cheap.
a UV system would work, but since you will be dealing with widely variable and unpredictable water qualities, the pre-filtering may end up more a hassle than is necessary.
Chemicals are alright, but be aware of how hot your car gets, and the expiry dates.
