コンダラ is a slangy word for a man-powered "land roller" to flatten the grounds, like this one.
Although the "correct" name of this tool is "(整地【せいち】)ローラー" or something, there are a few high school students who actually call this コンダラ. Even Japanese Wikipedia has the entry for コンダラ.
Yes, I'm only half kidding. What is really said in that song is "思い込んだら" (=if (you are) determined), not "重いコンダラ".
But you are not the only person who misinterpreted it as 重いコンダラ.
Actually there were great many native Japanese people who heard that song and wondered, "What's コンダラ? It must be something heavy, used by little leaguers? Oh I got it, it's the name of that roller!"
This error became a very famous joke, repeated on the radio and TV many times in the past. And some people actually started to call this コンダラ. And now, at last, there are people who call this just コンダラ and don't know why it's コンダラ!
You can show the picture above and ask "what's the name of this?", and I'm sure many people will answer "definitely it's コンダラ", jokingly or seriously. The following is a real question made by a native Japanese speaker who wanted to know the etymology of コンダラ.
And you can read the article コンダラ in pixiv百科事典, which describes this word rather seriously.
Edit: Putting the joke aside, 「思い込む」 here may be a bit confusing. In this context, 「思い込む」 means "strongly determine (one's way of living)", not "one's (erroneous) assumption".