404 Errors in Webmaster Console

What to do with these pages?

[rowtc2]

Hello , i have in Webmaster Tools 25.000 pages with 404 errors. I have changed my site URLs without a 301 redirect ,one month ago,because i have don't know this option. These pages are not in Google index when i search for each of them.
I can't block in robots.txt because there are not in a specific folder;
- old URL www.mysite.com/specificword1-variable.html
- new URL www.mysite.com/specificword1-variable-specificword2.html
I have entered these 25.000 errors pages in robots.txt,to not be crawled by Google.
What should i do with these pages for not see errors on my site by google (my traffic is decreasing and this could be a reason)? It is sufficient to block by robots.txt?

[buckworks]

Get busy and start making proper 301 redirects from the old URLs to their new equivalents, as many as you can.

Give priority to redirecting old URLs that have inbound links from other sites.