Ah--when you wrote "lectures / classes" I assumed an indoor setting. A lot of what I wrote won't apply so much.
I haven't done very much outdoor recording, but I have to say, 20 feet seems really distant no matter what type of microphone(s) you're going to use. I just can't imagine getting good results from farther than maybe 5 or 6 feet away--and even that assumes dependably quiet surroundings. The farther you get from your intended source, the more vulnerable your recording will be to interfering sound from all other sources in the neighborhood. (Who
are the people in your neighborhood?)
Also, wind has always been a big problem whenever I've done any outdoor recording, especially because I'm not "geared up" properly for it. Professionals who record dialogue outdoors always have to make very substantial investments in windscreens. Not using them is like asking for many of your recordings to be ruined. Again, though, all other things being equal, if there's some residual noise due to wind in a recording, then the closer you were to your sound source at the time, the less that amount of wind noise will detract from hearing what was said.
Definitely have a look at Rycote on
https://rycote.com--they have high-quality solutions for all kinds of stereo arrangements as well as individual microphones. Their U.S. distributor is Redding Audio (
https://www.reddingaudio.com). [Edited later to add disclosure: Redding is also Schoeps' U.S. distributor, and I have a long association with Schoeps; also, Scott Boland, who runs Redding Audio, is a friend.]
Gutbucket has an interesting suggestion: boundary-layer microphones. Those are most often omnidirectional--and if so, they'll be pressure transducers rather than pressure gradient (velocity) transducers, so they'll be inherently far more resistant to wind noise than any directional microphone--like by ~20 dB or so. Boundary mounting also gives a microphone the peculiar property of picking up direct sound with 3 dB greater sensitivity than diffuse sound, as Gutbucket mentioned--not a huge difference, but it can help. I need to put a disclaimer on this idea, though, since I've never tried it myself (and now I really want to; I have a pair of Schoeps BLM 03 Cs that I've only ever used indoors). Also, if the person or the people will be walking while talking, you couldn't use that approach at all, I don't think.
I really would like to see you get suggestions from someone who records outdoor dialog and/or effects professionally. Shotguns are definitely used in this application, since the "multipath" problem (different path lengths, and therefore different arrival times, for the same sound arriving at different angles) is fundamentally less severe outdoors. But a 20-foot recording distance just doesn't seem workable to me with any type of microphone I've ever heard of. Is there any chance that you could get significantly closer--or maybe put something like a Tascam DR-10L (
https://tascam.com/us/product/dr-10l/top) on the person doing the talking, and match the sound to the picture later on?
If these recordings are at all important (even if only to yourself), I think that you'd want to do some serious testing in advance--rather than just imagining (or asking other people) what it should be like, then finding out that reality was all rude and didn't behave as you thought it would. Reality can have bad manners in that regard.
--best regards