Maybe not such a disaster. Over such a long term period you'll probably do OK. The fall in the FTSE100 has taken us back 31 months, but that's only a fraction of your planned investment duration.
it could be that building a long-term portfolio quickly is not the best entry technique - feeding money in each year with an annual cap might ensure you don't jump in all at once at what turns out to be bad timing. Also, when we get a fall like we're now seeing, you'll usually still have cash left so you can go in bigger when shares are "cheap".
Being a 100% shareholder may also not be a great plan. So often, when shares go down, even "good" shares go down. But at times like these, the big players haven't usually just taken money out of shares, they usually rotate it into other investment areas - currencies, bonds, gold, commodities, property etc. You might think about some long-term exposure to these too, or keep cash aside to buy it when needed as a short-term hedge.
Don't forget, if you own your home, you're already heavily long on the UK residential property market.