Nobody can know what the Market will do over a short time period with any consistent certainty. I've seen it a million times where someone insists it is time to buy gold or oil or biotech and their odds of being correct are, at best, 50/50. The trick salesmen use is to leverage the few times they are correct as collateral for validity in managing your money. You will likely never see a list of the times they were wrong next to the times they were correct.
The "long run" is still a guess. However, the odds may be a bit more in your favor based on historical evidence. There was a time when I would have bet substantially that I wouldn't lose money over any 10 year period in the S&P 500 Index. Then, 2000 to 2009 came along and showed me something very different. Yes, you could come out a net loser over a decade which, to many, is considered a long term. Imagine you began contributing to a 401k in 2000 just after graduating college and after the Great Recession, figured out you would have been better off in cash rather than putting it all in a single index fund. Or, even more emotionally difficult, imagine you retired in 2000 relying on income from your investments. If not diversified well, you could still catching up!
Nobody can know the future. However, I certainly hope the next seventeen years are not similar to the last seventeen. If so, you certainly should be investing carefully.