I have a small website and usually I have a very normal number of visits/pageviews/length of visit such as 20/200/5 minutes but today I did a bit more advertising in my area and I had 40/1500/20 minutes. The number of visits is normal but can you think of anything that would cause that crazy number of pageviews and visit length. Could some sort of bot have caused this?
What do you mean by advertising in your area? What type of advertising did you do exactly? The link time on pages can simply be someone leaving their browser open or switching tabs in their browser.
You should drill down in Analytics to see the city where the visits originated for the day with the high number of pageviews. Then add a metric to see the source was it direct, or a referral? Did you tag your links for the campaign to register in Analytics so you could track the campaign?
20 minutes visit length: Average time on page? Or is this the longest seen time on page of one visitor?
Pageviews: Is this the number including non-unique pageviews? Or the unique pageviews (same visitors)? What date ranges did you compare?
You have some options available in GA to nail it down or to verify the traffic spike is related to your advertising: (If you did not setup campaign tracking)
Content -> Site Content -> All pages:
You might also try "Audience" -> "Visitors Flow" after you specified the date range to compare from the beginning of your traffic spike. Default is country/territory here, play with Traffic sources/Traffic type.
Or go to "Audience - Technology - Browser&OS/Network" and see if there are certain technology changes. In this context you might also go and choose a date range you compare to a similar (day/week) range before the spike, choose the "Comparison" button right after the advanced filters link and select "Visitors" and "Avg. Visit Duration", to see differences in usage.
To me it seems to be a bot. I too have a site whose bounce rate has become, 1%-2%, before it was 40%-50% and avg pageviews increase to 5.
I overcame this issue by using captcha in my contact form and forum, as it does not allow the bots to crawl the site. I would suggest to go through your site, and see the loopholes through which bots can crawl.
The most common places for bots to attack are Contact form, forum( to put backlinks), comments or any part of site from where bot can crawl and send email or put backlinks.